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"The influence of impure water on the spread of cholera"

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(July 1866): 182-90
MacMillan's Magazine

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In the December number of this Magazine I gave an account of an investigation into the probable causes of the cholera outbreak in St. James's, Westminster, in 1854, stating the reasons which led the Committee of Inquiry to the conclusion that the outbreak was in some manner attributable to the use of well-water contaminated by matter from an adjoining cesspool, and also showing that, for three days immediately preceding the outbreak, there had been poured into this cesspool the diluted dejections of an infant registered as having died from exhaustion after diarrhoea; a fact to which, with reference to its bearing upon a theory as to the mode of propagation of cholera, I must presently call further attention.

Meanwhile, apart from any theory respecting cholera, there can be but one opinion as to the extreme unpleasantness of any such discovery as that of a connexion between a well and a cesspool, suggestive, as it was, of a similar state of things in other parts of London; for it came to the knowledge of the Committee, while prosecuting their researches, that cesspools abounded even in a comparatively well-regulated parish like St. James's. Among other evidence to this effect it was stated that "when Derby Court, Piccadilly, was pulled down to clear a site for the Museum of Economic Geology, no less than thirty-two cesspools had to be excavated" (St. James's Cholera Report, p. 57). There might not perhaps often be only "two feet eight inches of horizontal distance" between a cesspool and a well; but through such soil as that of London the percolation is necessarily very extensive. Nor was the matter at all mended, but rather in those days made worse, by the fact of the cesspools being mostly joined by drains to the sewers.

The Broad Street cesspool, for instance polluting at once the river through the sewers and also a well through the soil, was a gross superfluity of nuisance.

Some excuse indeed might have been alleged in 1854 for the retention of cesspools in London, provided they had not been put in connexion with the sewers, which at that time discharged their contents not very far from the source of the water-supply of one of the public companies. But, now that all the public waterworks are beyond the flow of the tide, and the outfall of the sewers is several miles below London Bridge, with a prospect of the sewage being soon altogether diverted from the river, let us hope that the extermination of cesspools is going on all over London at least as rapidly as in St. George's, Southwark; the authorities of which parish, as I have been told, have excavated 4,000 of these obnoxious pits during the last four years.

Not that even the total disappearance of cesspools would render the use of the vast majority of the metropolitan wrells unobjectionable. Only such of them as are sunk through the clay to the chalk, and of course carefully secured against the entrance as well of landsprings as of soil-soakage of any kind, can be wholly free from suspicion. But a deep public well is a rarity in London. Most London wells, sunk only to the clay, and having the bricks which form their sides laid dry, without mortar or cement, in order that the water may readily enter, must by their very nature admit of percolation from the surface and from the various strata which surmount the clay. What then with churchyards, gas, and muck of every description, including even "enormous quantities of black beetles" (St. James's Cholera Report, p. 76), such wells can hardly escape contamination of some sort, and should accordingly share the inevitable fate of the cesspools.

The wells, however, will probably die harder than the cesspools; for it is difficult to persuade the general public of the deleterious character of water which is pleasant to the eye and the taste, whilst its pollution, besides being imperceptible to the drinker, is the result of a process at once removed from observation and requiring some little intelligence to understand: and, on the other hand, it is not at all difficult, especially in loathsome courts and alleys, to see at a glance the unsatisfactory condition of the "butts" which serve as receptacles for "the Company's water," the only alternative, of course, to the pump-water. Enough has recently been written and said on this point to render it unnecessary here to dwell upon it further than to remark that, during the St. James's cholera outbreak, in a district by no means of the lowest class, the state of the cisterns was often alleged by the inhabitants as a reason for sending to the pump. No wonder, then, if people in miserable "rents" are glad enough to avail themselves of the sparkling water of any well within reasonable distance. Indeed it is sometimes the case in such places, as has lately been shown by Dr. Jeaffreson, that there is positively no butt or cistern at all. Under these circumstances we must perhaps for the present rest content with knowing, on the authority of the Registrar-General, that "the London pumps have been placed under inspection" and on the authority of the Prologue of the Westminster Play, that underground railways and main drainage works are intercepting the sources of their supply. But though such wells for the most part must be of a dangerous character, some special contamination, apart from an habitual pollution generally detrimental to health, may be necessary to connect them with the spread of any specific disease.

That an exceptional contamination is needed to enable them to propagate cholera is rendered highly probable by some of the facts which came under the observation of the St. James's Committee:-"If," says their reporter, Mr. Marshall, "we may found an argument upon the fact that other well-waters in the same neighbourhood, containing much the same recognisable organicand inorganic impurities, did no harm, and that the Broad Street water itself, in 1832 and 1849," and he might have added in 1853, "when it was probably also extensively used for drinking, and must have been charged with very similar general impurities, produced no perceptible deleterious effects, we may admit the possibility that its apparently fatal influence in determining the brief but severe explosion of 1854 was owing not to a general impurity, but to the temporary existence of some special contamination" (Report, p. 92). Likewise, in noticing this outbreak and the verdict of the Committee that the pump was in some manner the cause of it, the medical officer of the Board of Health, Mr. Simon, whilst admitting that "the evidence collected by the Committee strongly tends to justify their opinion," calls attention to the fact that, "bad as was the produce of the Broad Street well-containing the results of organic decomposition filtered through but scanty thickness of surrounding soil-this quality of water was not peculiar to it; as generally through London such must be the condition of superficial well-waters;" which waters, as already observed by the Committee, had not in other cases been proved to have had any share in the spread of cholera. Wherefore, adverting also to the researches of Dr. Pettenkofer, who "found himself unable to attribute to the well-waters any causative relation to the epidemic" at Munich, Mr. Simon concludes that these facts "give to the imputed operation of the Broad Street water at least an exceptional character " (Report to the Board of Health, 1856, p. 9).

Such from the first had been the opinion of Dr. Snow, who consistently maintained, not merely that this well must have contained "the results of organic decomposition," but that, at the time of the outbreak, its pollution, as compared with that of other superficial wells, must have been of an "exceptional character;" though he did not, as will presently be seen, admit it to be an isolated exception, even among wells, in its connexion with cholera. "The reason," he used to say at the meetings of the Committee, "why the water of this pump produced the great outbreak is, I feel confident, that the evacuations of one or more cholera patients found their way, by some means, into the well" (St. James's Report, p. 119). To the swallowing of such evacuations, at first exclusively through the water, and afterwards both through the water and in various other ways by reason of the multiplication and spread of the morbific matter, he attributed the very great mortality within the circumscribed area of which the pump was almost the exact centre.

I need not here recapitulate the stages by which Dr. Snow's inculpation of the pump, at first disallowed, came gradually to be respected, and at last unanimously accepted by the Committee, or the means by which the well, at first pronounced "free from any fissures," was found to be connected by "furrowed channels" with a cesspool. Passing over these matters, as requiring no further exposition, the point now to be considered is the probable cause of exceptional contamination. On this point Dr. Snow's hypothesis required evidence, not only that "the evacuations of one or more cholera patients" found their "way into the well," but also that at least one case thus concerned in the pollution of the well preceded the great outbreak by an interval long enough to allow time, both for the process of pollution, and also for the incubative period of the disease. It was on the evening of Thursday, August 31st, that the outbreak began: and what is the fact brought to light some time after Dr. Snow's prediction of its probable discovery, and after all doubt had disappeared from the mind of the Committee respecting the influence of the pump in producing the outbreak? It is found that an infant was seized with diarrhoea on Monday August 28th, and that its dejections till Wednesday, the 30th, steeped in water, were thrown into the cesspool the direct connexion of which with the well is proved beyond cavil or dispute. Such remarkable agreement between theory and fact must not, I apprehend, be lightly dismissed as a mere coincidence.

That there should be difficulties in the way of accepting the case of this infant as a complete explanation of the cause of the outbreak is no more than might be expected. One difficulty I myself was the first to suggest.

Why, I asked, did three subsequent cases of undoubted cholera in the same house fail to sustain the outbreak through the same channel of communication with the well?-In my December paper on this subject I gave reasons for believing that in these cases the excretions may not have found their way even into the cesspool. But a friend suggests-and not without reasons derived from accurate knowledge of the circumstances- that, if they did enter the cesspool, they may not have reached the well.

But the child's illness, it is urged, was not choleraic; it is registered as diarrhoea.-The force of this objection is lessened by medical testimony of considerable weight. "There is evidently," says Dr. Gull, "in some minds a disinclination to regard any case as cholera, unless it manifests symptoms of marked intensity; but that the diarrhoea which prevails when cholera is epidemic to the same cause as cholera itself, is to be inferred, not only from its clinical history, but also from other circumstances" (Report to the College of Physicians, 1854, 2d Part, p. 124). The same opinion is expressed by the Committee of the Medical Council:-"When diarrhoea and cholera prevail together epidemically, are they (with differences of degree) the same disease?" This question must now doubtless be "answered affirmatively " (Report to the Board of Health, 1855, p. 62)

If it should still be asked, as it has been continually asked of me, whence and how did the infant catch the disease, I can only reply that I do not know, and that it is not at all necessary for the purpose of a local investigation to inquire. But I must remind the reader that Dr. Snow did not limit the propagation of cholera to water as the sole medium of communication. The morbific matter, if such as he supposed, may be conveyed in clothes, on a person's hand, in the air as a minute particle of dust, nay, even by flies. The infant in question was brought up by hand; and no one knows what may not be in London milk.

But it is impossible, say some members of the medical profession, that cholera can be at all propagated by the swallowing, whether in water or in any other way, of choleraic discharges, the theory being contrary to sound doctrine concerning the pathology of the disease.-It needs not, however, a professional acquaintance with cholera literature to be aware that this theory has been defended as well as attacked on pathological grounds. Of course, on such a point, I must be content to listen respectfully to a controversy in which I can have no part, unless the disputants on either side should condescend to appeal to external evidence, to which step sooner or later both sides are generally reduced. But I may remark that two recent utterances respecting the mode of propagation of cholera seem to show that pathological objections to the theory in question do not press very heavily upon the minds of some of the highest authorities. "For all practical purposes," says the Registrar-General, "it may be assumed that the discharges of patients in the epidemic, either casually touching the mouth, or entering in dust or vapour through air or water, induce diarrhoea or cholera in a certain proportion of those exposed to their influence (Quarterly Report, October, 1865). And a similar doctrine is reported by the Times' correspondent at Naples, in a letter dated October 11, as being held by "one of the most eminent of the Neapolitan physicians," Dr. Thomassi, "who expresses the opinion that the mode of communication of cholera is not by contagion from the skin or exhalations from the lungs, but from the materia ejected by the patient, the virus of which may be extended by water if the materia has been thrown near water-courses or cisterns."

The Registrar-General attaches great importance to this theory, and speaks of "the detection since 1849 of the mode of propagation of cholera" as being "among the greatest triumphs of medical science." A like estimate of its importance, as supplying definite aim to works of sanitary improvement for the prevention of cholera, and also introducing intelligible purpose into precautions taken by individuals, was thus expressed some years ago in the Lancet by Mr. French, the surgeon of the St. James's Infirmary: "When this" (Dr. Snow's) "doctrine, respecting the mode in which cholera is communicated, becomes comprehended by Secretaries of State and generals commanding in chief, as is the household word 'vaccination' then 'outbreaks' of cholera- i.e. large numbers of persons attacked at once in a district (a phenomenon well known in the history of the disease)-will become rare events" (Lancet, July 24, 1858). This statement was made on the occasion of the death of Dr. Snow, of whose investigations on this subject a brief account may, at the present time, be interesting, and perhaps useful, to the public.

It was in 1848 that Dr. Snow, having already, with many other members of his profession, become convinced, from examination of the history of cholera, that it is propagated by human intercourse, was led, by study of the nature of the disease, to the conclusion that it is "communicated by something that acts directly on the alimentary canal;" and he then suggested "the excretions of the sick" as the most likely vehicle of the poison, which, in his opinion, to produce cholera must be swallowed. This hypothesis, besides being in accordance with all that had been urged in support of the contagiousness of the malady, had the further recommendation of supplying an explanation of its apparent eccentricity. It did not, however, more than other theories, seem to admit of demonstration, except in one way, which had not yet been investigated. "None of the early investigators of cholera appear to have imagined that the specific poison ever found entrance by means of drinking water" (Dr. Parkes on Practical Hygiene, 2d Edition, 1866, p. 60).

Applying himself to this branch of inquiry, Dr. Snow was only able, in 1848, to allege that certain towns, the inhabitants of which drank water polluted by sewage, had suffered severely from cholera in 1832, and, in particular, that in certain districts of London, supplied with water from the Thames and the Lea, the mortality in that year from the same disease had been greater than in other parts of the metropolis differently supplied.

But, in 1849, one of those anomalous outbreaks, which obviously suggest a local cause, took place in Horselydown, where of two rows of houses, placed back to back, using the same drain, and having the same relative position towards an open sewer, one row lost eleven of its inhabitants by cholera, several others being not fatally attacked, in a few days; whilst the other only lost one, and had but one not fatal case. The only difference which the Assistant Surveyor for the Commissioners of Sewers could find between the circumstances of the two courts, was that the people of the former used a well to which slops of dirty water, poured into a channel in front of the houses, had access. The outbreak was preceded, with an interval of a week, by two cases, which each lasted several days, and, as stated by the doctor who attended them, "the water in which the foul linen would be washed would inevitably be emptied into the above-mentioned channel" (Snow on Cholera, 1st Edition, p. 14,1849).

Simultaneous with this outbreak was another, in Albion Terrace, Wandsworth Road, in which, in the seventeen houses constituting the terrace, twenty persons died of cholera in a fortnight, four or five others being fatally attacked after flying from the place. The fatal cases were distributed over ten out of the seventeen houses; and a doctor who attended many of the cases stated that "attacks occurred in the other seven houses, with the exception of one or two that were empty, or nearly so." The immediate neighbourhood was altogether spared. The only circumstance peculiar to this terrace was its water-supply, derived "from a copious spring in the road, the water of which was conducted by a brick barrel drain to the back of the houses and then flowed right and left, to supply tanks in the ground behind each house." The cesspools were full, and some of the overflow drains choked up, "one having no bottom, or one so soft that it could be penetrated with a stick; and it crossed, at right angles, above the earthenware pipe of the water tank, the joints of which were leaky, and allowed the water to escape." In one case the Surveyor "found a pipe" for bringing surplus water from the "tanks communicating with a drain from the cesspool; and he found a flat brick drain laid over the barrel drain which brought the water from the spring." This state of things, bad enough at any time, was made considerably worse by the bursting of the "flat brick drain" during a storm on July 26th. The first case began on July 28th; the second (not fatal) on July 30th; two more on August 1st; two on August 3d. The attacks were numerous during the following three or four days, after which time they diminished in number (Ibid, 1849, p. 18).

Having inquired into the circumstances of these local outbreaks, Dr. Snow turned his attention to the general mortality from cholera throughout London, which he found, as in 1832, was by far the greatest on the south side of the river. "The deaths from cholera in this district, which contains a very little more than a quarter of the population, have been more numerous than in all the other districts put together" (Ibid 1849, p. 23). This calculation was based on the reports of the Registrar-General, which showed that, during the month ending August 25th, "out of the 7,266 deaths in the metropolis, 4,001 occurred on the south side of the Thames." The South London drinking water, so far as it was supplied by public companies, was at that time exclusively derived from the Thames at and below Vauxhall, whilst other sources of supply were used by extensive districts on the north side of the river.

Believing that he saw in these facts some confirmation of his theory-which of course attributed the excess of mortality in South London to the contamination of the Thames by cholera discharges entering it through the sewers- Dr. Snow ventured, in the autumn of 1849, to publish his views, which he had previously made known, in conversation only, to various members of the medical profession. He did not claim for them the credit of actual demonstration, but advanced them as containing, in the existing state of knowledge upon the subject, "a greater amount of probability in their favour than any other views."

In the interval between the disappearance of the cholera in 1849 and its return in 1853, much additional information came to light respecting the influence of polluted water in propagating cholera during the epidemic of 1849; and Dr. Snow, availing himself of the observations of other medical men, and corresponding with persons in various parts of the country, continually drew attention to the subject. It appeared that in Rotherhithe, Salford, Ilford, and Locksbrook, outbreaks of cholera had been traced, with more or less probability, to wells contaminated with faecal matter; and an inquiry, set on foot by Dr. Snow in 1849, into the circumstances of an outbreak at the village of Kewburn, near Newcastle, where one-tenth of the population died, afforded strong grounds for attributing the outbreak to a similar cause. It was shown by Dr. Shapter that at Exeter, in 1832, when the public water-supply was derived from the river at a point where it received the sewage, the mortality from cholera was seventeen times greater than in 1849, when the source of supply had been changed to a locality beyond the influence of the tide. At Nottingham the deaths in 1832 were twenty-two times more numerous than, in 1849; which difference wasattributed by the Local Sanitary Committee to the extension, since 1832, of pure water to the poor, who in that year had been the principal victims. On the other hand, at Hull the mortality in 1849 was six times as great as in 1832, apparently owing to "a more plentiful supply" of contaminated water. In 1832, Glasgow suffered very severely from cholera on both sides of the Clyde, from which its water-supply was then on both sides derived; but in 1849 the south side, then supplied with water from the neighbouring hills, "furnished comparatively a small number of cases, whilst in other parts of Glasgow, still supplied from the Clyde, the epidemic was very severe. "Birmingham, Bath, Cheltenham, and Leicester, all with a supply of water free from connexion with the drains and sewers, have nearly escaped the cholera in every epidemic" (Snow on Cholera, 2nd Edition, 1855, p. 98).

It is the cumulative testimony of these instances, with others that need not be mentioned, which gives weight to the charge of the preponderating influence of impure water in the propagation of cholera. Some of them, as was shown by the late Dr. Baly, may be open to objection; but he admits that this alleged influence is "supported" by evidence of a striking and unexpected kind," and concludes that "the subject needs and is likely hereafter to receive more systematic investigation" (Report to the College of Physicians, 1854, Part 1, p. 214).

It had already begun to receive "more systematic investigation," the prosecution of which was facilitated by an important change which had taken place since 1849 in the water-supply of some of the South London districts, the Lambeth Company having removed their works in 1852 from opposite Hungerford Market to Thames Ditton, thus obtaining a supply quite free from the sewage of London, whilst the South-wark and Vauxhall Company still continued in 1853 to draw their supply from near Vauxhall. The effect of the change on the part of the Lambeth Company was at once apparent on the return of the cholera in 1853, and attracted the attention of the Registrar-General, who commented upon it in his returns; and Dr. Snow records some startling particulars. Thus, "in 1849, the parish of Christchurch suffered a rather higher rate of mortality than the adjoining parish of St. Saviour's; but in 1853, while the mortality in St. Saviour's was at the rate of 227 to 100,000 living, that of Christchurch was only at the rate of 43. Now St. Saviour's is supplied entirely by the Southwark and Vauxhall Company, and Christchurch chiefly by the Lambeth Company." Again, "Waterloo Road, first part, suffered almost as much as St. Saviour's in 1849, and had but a single death in 1853 ; it is supplied almost exclusively by the Lambeth Company." But this contrast becomes still more striking when traced in certain districts where " the pipes of each company go down all the streets and into nearly all the courts and alleys, some houses being supplied by one company and some by the other, in many cases a single house having a different supply from that on either side." With great industry, Dr. Snow, by order of the Registrar-General, examined the death statistics in these streets, and found that down to the 12th of August, 1853, whilst deaths from cholera had occurred in thirty-eight houses supplied by the Southwark and Vauxhall Company, deaths had only occurred in four houses supplied by the Lambeth. Moreover, "of all the deaths from cholera throughout London in those four weeks, more than half took place among the customers of the Southwark and Vauxhall Company" (Snow on Cholera, 2d Edition, 1855, p. 80). Similar results were observed in 1855, when it also appeared that the customers of the Lambeth Company enjoyed greater immunity even than most of the districts north of the Thames. A more complete investigation, instituted by the President of the Board of Health, and embracing the whole period of the epidemic of 1853-4, resulted in the discovery that among an equal number of the customers of either company, the deaths among those of the Southwark and Vauxhall Company were three and a half times more numerous than among those of the Lambeth, which fact the reporter, Mr. Simon, "ventures to believe will be accepted as the final solution of any existing uncertainty as to the dangerousness of putrefiable drinking water during visitations of epidemic cholera" (Report to the Board of Health, 185G, p. 9).

Since 1854 the Southwark and Vauxhall Company has likewise removed its works to Thames Ditton; so that there is reason to hope that on the return of the cholera the whole of South London may be comparatively lightly visited; though there is still some, however inconsiderable, danger from the sewage of towns above Teddington Lock.

Of course, even when the influence of polluted water upon the prevalence of cholera is admitted, different theories may be advanced as to the way in which it acts. Some will say that it predisposes or prepares the system to be acted upon by some unknown cause existing in the atmosphere or elsewhere. But Dr. Snow held that "opinion cannot long halt here; and, if the effect of contaminated water be admitted, it must lead to the conclusion that it acts by containing the true and specific cause of the malady" (Snow on Cholera, 2d Edition, 1855, p. 110), which, in the case of the Thames, he supposed to be transmitted from cholera patients through the sewers. He appealed, in support of his doctrine, to the fact of several of the above-mentioned outbreaks having been immediately preceded by isolated cases of cholera in houses of which the drainage found access to the inculpated wells. Doubtless, in some of the earlier instances, the evidence on this point was defective. It could scarcely be otherwise, however true the doctrine for Dr. Snow, though wonderfully industrious, could not be everywhere during an epidemic; and in 1849 the attention of local investigators, even of the medical profession, had not yet been sufficiently directed to the subject. But fortunately Dr. Snow happened to live near to the scene of the most remarkable local outbreak of 1854, and was therefore able both to devote considerable time himself to the investigation of it, and also, as a member of the Committee of Inquiry, to secure at the hands of his colleagues, most of whom lived in the same neighbourhood, a diligent collection and an attentive consideration of the facts bearing upon his hypothesis.

Yet, even with these advantages, the last link in the chain of evidence was long missing, and narrowly escaped being altogether overlooked. It is therefore very probable that in other cases deficiency of evidence may have been due to incompleteness of inquiry; and we may hope that for the future no pains will be spared to make such investigations as complete as possible.

Meanwhile those who believe, on the evidence already adduced, that it is when contaminated by cholera-discharges that water propagates the disease, will be of opinion that the character of the operation of the Broad Street water in 1854 ceases to appear "exceptional" when viewed in connexion with the rule of the Thames ; on the nature of which rule, from the very fact of its being an exception (though by no means a solitary one) among wells, it throws a good deal of light. For, if the fecally-polluted water of the Thames (below Vauxhall) has always been true to its fatal traditions, why does only here and there a fecally-polluted well, and only during one of several epidemics the selfsame well, manifest "a causative relation" to cholera? In what way did the law of the Thames embrace the exception of the wells, unless by the inevitable admission into the river, through the sewers, of those choleraic discharges which would only find exceptional entrance into a well? It is certainly a point gained that the dangerousness of putrefiable drinking water during visitations of epidemic cholera has at length arrived at a final solution and, even if that be all we know on the matter, to Dr. Snow belongs the merit of the discovery. But why is the dangerousness, as shown by the wells, notoriously incommensurate with the putrefiableness. If Dr. Snow be right in his hypothesis, that water to produce cholera must be specially contaminated by choleraic discharges, then we need not be at all surprised at the separableness of putrefaction and danger; and still further credit accrues to his memory, with, still greater benefit to the community. But if on this point he be wrong, then we are still very much in the dark respecting the mode of propagation of cholera, the mystery of which will by no means have been solved when a few or even a great many more local outbreaks shall have been traced, as doubtless they will be traced, to the influence of polluted water.

We shall still require to know what gave to the "imputed operation" of the water its "exceptional character." One such local outbreak, apparently of a kind very favourable to minute investigation, has already occurred in this country. In the autumn of last year, not long after the importation of cholera from the Mediterranean, we read in the papers of a severe outbreak at Epping, concerning which we as yet know nothing beyond what we could gather from the press reporters, who, after a momentary inclination to the traditional hypothesis of a "dung-heap", finally settled down into the opinion that the mortality was due to the use of a foul well.

They did, however, likewise inform us that the Government had sent down an inspector to inquire into all the circumstances of the case, whose report, one would think, should before now have been made public. Whether his conclusions coincide or not with the ultimate verdict of the press remains to be seen.

And, if they do so coincide, we shall still be anxious to know whether any light has been thrown upon the cause why at that time one only of all the filthy wells throughout the country should produce or spread cholera, and why also this particular well should have waited, before manifesting a "causative relation" to cholera, till that disease had been imported into England by "human intercourse."

It is easy, however, to see how the conveyance of cholera to remote distances by human intercourse, though well established as a general truth, may yet in particular cases evade all attempts at actual demonstration, especially when the manifestation of the disease at the point of arrival is separated by a considerable interval of time as well as space from its apparent point of departure. When the interval of time is short, and the suspected carrier of the disease, himself the first victim at the point of arrival, is known to have come within forty-eight hours from an infected neighbourhood, we may assume that he swallowed the poison before setting out on his journey. Dr. Snow supposes the period of incubation to vary from twenty-four to forty-eight hours. The man who left Botterdam on Sunday morning, the 22d of last April, and was seized with cholera in the train at 11 a.m. on Monday the 23d, on his way to Bristol, may easily have brought the disease with him in this way. The fact of its failing to spread at Bristol may be due to care both in the management of the patient and in the disposal of his clothes and discharges at that town. Thanks to the Registrar-General, people have now been widely put on their guard respecting these matters, and we may hope that special precautions will everywhere be taken with regard to first cases. But no such precautions, we may be tolerably sure, were or even could be observed among the German emigrants on board the Atlantic steamers, so that when once the disease appeared among them it had every facility for spreading. Each new case would multiply the facility in such confined space; for, next to the danger arising from water poisoned by cholera-discharges, there is no greater risk of such discharges being swallowed than that occasioned by extreme overcrowding, where people eat, drink, and sleep, all huddled together in a small place. That these emigrants brought the material cause of the disease from Botterdam can scarcely be doubted. How they brought it, as the interval seems too long to be accounted for incubation, does not appear. But the mere absence of evidence on this point proves nothing against a theory which recognises the possibility of the seed of the disease being carried any distance in clothes or bedding, and therefor having to wait some time before they happen to be accidentally swallowed. One thing is certain, that they left no trace on their way-at Hull, for instance -which seems to indicate that cholera does not readily develop itself apart from a case in active operation, when of course the chances of swallowing the poison are largely increased, and, with great overcrowding or water contaminated by the discharges, a severe outbreak is the certain result. Each outbreak necessarily heightens the danger of the disease being extended in all directions; and yet it is wonderful and very satisfactory to observe how little mischief the most violent local outbreak often produces even in streets and houses immediately bordering upon its own well-defined boundaries. This seems to show that the danger arising from the discharges being carried in the air as dust is not very considerable.

Of the more mysterious general conditions which may be necessary to render the spread of cholera possible at one time and not at another, it is not for me to speak, except reverently to recognise their dependence upon the decrees of the Divine Will. Subject to those conditions, whatever they may be, there are evidently less recondite conditions which determine its actual propagation by human intercourse and its greater or less development in particular places. To these I have confined my attention; and if what I have written upon the subject shall tend in any degree to moderate some of the terrifying notions which prevail concerning the secondary causes of cholera, I shall be content.

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