In order that the reader should grasp the reasons for the various principles which are to be inculcated in this work, it is essential that the effects of insanitary conditions in producing disease should be discussed in some detail.

Diseases Conveyed By Impure Water

It is hardly necessary to discuss the effects of an inadequate supply of water for domestic purposes. Such a condition leads to personal uncleanliness, to dirty houses, and to imperfect flushing of drains, - in other words, to a state of affairs in which not only are the specific germs of disease harboured and propagated, hut the powers of resistance against them considerably lowered. The main point to which attention is here called is that certain specific diseases are spread by the contamination of drinking-water with their germs. (Chief among them are the infectious maladies characterized by diarrhoea - to wit. cholera, typhoid fever, dysentery, and summer diarrhoea. We now believe that these diseases are due to specific micro-organisms, and in the case of the two first-named our knowledge of the disease-germs is detailed and accurate. We know that they pass from the bodies of those affected by typhoid or cholera with the excreta, and that they are capable of living for some time outside the body. They are thus disseminated in the sewage, and where defects exist in drains or cesspools they may escape into the soil and contaminate surface-wells. Streams from which water-supplies are drawn may be infected, or even the main water-supply of a town. Numerous instances are on record in which disastrous outbreaks of cholera and typhoid have been traced to the contamination of public water-supplies by sewage containing the excreta of persons suffering from these diseases. In the cases of dysentery and simple diarrhea the evidence is strong in the same direction, i.e. that certain outbreaks have resulted from the consumption of polluted drinking-water, but it is not generally thought that this is the principal way in which these diseases spread. There is reason to believe that in malarious districts, drinking-water forms one way in which the specific parasite of malaria gains access to the system.

The pollution of water by organic and even focal matter, apart from the germs of specific disease, is a different question. Until recent years such contamination, as shown by chemical analysis, was the only available evidence against a suspected water-supply. It still remains evidence of value, since it indicates possible contamination by specific disease-germs, but bacteriological examination now constitutes an indispensable adjunct to chemical analysis. It is possible that dissolved or suspended organic matter in drinking-water may be a direct cause of diarrhoea or other disease, though it is difficult to exclude the effect of the microbes which must be present under these conditions. The main water -borne disease to be dreaded in this country is typhoid fever; cholera would in case of its introduction be equally to be feared. The other zymotic diseases are rarely, if ever, conveyed by water.

It must further be observed that certain mineral impurities in water are capable of causing disease. Excessive hardness of water may give rise to dyspepsia; the prevalence of calculus in certain districts has been ascribed, though on little evidence, to peculiarities in the water; goitre very probably depends on some as yet unidentified mineral ingredient in water, though this seems certainly not to be carbonate of lime. The chief mineral impurity to be guarded against in a domestic water-supply is lead, which, when continuously absorbed - even in minute quantities - induces lead-poisoning with its various troubles - amongst others anaemia, constipation, severe colic, and local paralyses. The source of the mischief in these cases is, of course, the lead used in plumbing-work, and it is to be noted that the degree to which different waters attack lead varies largely, the purer and softer waters, especially those containing free acid, doing so far more than the harder waters. It is believed also, as the result of experiment, that bacteria may play a part in the action of water on lead.

Lastly, the introduction of certain intestinal and other parasites, in the form of egg or embryo, is a possibility not to be lost sight of in considering water as a source of disease. This can only occur as the result of gross impurity, and the danger is probably far less in this country than abroad.

It results from the above considerations that much attention may justly be devoted to the purity of the drinking-water in a house. Purity of source is the first essential. Where the supply is derived from rivers or lakes, it ought to be filtered by the water company before distribution; but it must be remembered that filter-beds do not remove all or nearly all the micro-organisms from water, and may at times, especially during severe frosts, be very imperfect in their action. This is the case too during the first day or two on which a filter-bed is used. It is therefore a wise precaution to employ either boiling, or efficient domestic filtration, in all Domestic filtration will be dealt with in detail in a subsequent chapter, but the principles which should underlie it may be noticed here. The aim of filtration is to deprive water of foreign matters, and especially of those microorganisms which have been shown to produce disease. The bacteriological study of the subject has shown that this aim is not only unfulfilled in most of the old-fashioned filters in which we have been accustomed to repose such simple trust, but that, when they have been for any time in use, the filtering medium becomes so thickly impregnated with bacteria, all alive and growing, that it may positively add them to the water. Few filtering media are so fine as to exclude bacteria, and when they do so the process of filtration is so slow that considerable pressure has to he employed. Such pressure is fortunately available in most houses in the service pipe itself, or from the hydrostatic pressure of the column of water from the top of the house. High-pressure filters are the and safest for domestic use, but they nevertheless require frequent attention and cleaning if they are to deliver a supply of really sterilized water.

Where the domestic water-supply is intermittent, the storage of water in cisterns is necessary, and this is not undesirable even under a constant supply, since the latter may possibly fail in time of drought or when mains are frozen or burst. Unless proper precautions are taken, cisterns may be a source of danger. The ordinary form of cistern, imperfectly covered by a wooden lid, is not an arrangement to be recommended; the water is liable to various sources of contamination from air and dust, sometimes even from sewer-gas where the constructional arrangements are bad; drowned rats and sparrows are not unknown; periodical cleansing is frequently neglected, or performed at too-long intervals. The ideal cistern, as pointed out by the recent Commission on the East London water-supply, is a mere expansion of the service-pipe, perfectly closed except for a minute air-valve, and preferably of a conical shape, so as to be self-cleansing.