=Effects of an Insufficient Supply of Water.=—The influence on personal health is most baneful. Water is used sparingly for purposes of cleanliness, with the necessary results that cutaneous diseases become more common, and the whole body suffers; the linen is imperfectly and infrequently washed; the house becomes dirty; drains are imperfectly flushed; the streets are not cleaned; and the whole atmosphere becomes loaded with impurities. According to Parkes, it is probable that the almost complete disappearance of typhus fever from civilized and cleanly nations, is not merely owing to better ventilation, but also to more frequent and thorough washing of clothes. Insufficient cleansing of the surfaces of streets and of sewers, owing to a deficient supply of water, has a very important influence on the spread of enteric fever and epidemic diarrhœa. A heavy fall of rain often causes a rapid diminution in the prevalence of the latter disease. CHAPTER XIII. THE PURIFICATION OF WATER. When a public water-supply is provided, it may reasonably be expected to be furnished pure and fit for use; but this, occasionally is not so. The reports, for instance, of the condition of the London Water Supply, occasionally show that it is turbid and contains a slight excess of organic matter. This is especially the case when, after heavy rainfall, storm-water is brought into the reservoirs, and owing to deficient storage, sufficient time is not allowed for deposit. Rain-water always and other waters frequently require to be purified before use. =Methods of Purification.=—The only certain way of obtaining pure water is by =Distillation=; but this plan is scarcely applicable to water on a large scale. Furthermore distilled water is not so palatable as ordinary water. The distillation of water is more especially required on board ship, during long voyages. It should be followed by the use of some measure to secure efficient aeration. 2. =Boiling water= serves to remove the temporary hardness, and the chalk carries down with it a large proportion of any organic matter that may be present. Boiling deprives the water of its dissolved gases, and renders it flat; it is desirable, therefore, to aerate it by filtration or from a gazogene after boiling. All the microbes which are known to produce disease are destroyed by efficient boiling. Certain putrefactive microbes are more persistent of life, owing to the fact that they form spores, which are not killed at the temperature of boiling water. Tyndall showed that by boiling the liquid containing these spore-forming microbes on three successive days, thus giving time for the spores to develop into less resistant microbes, they could be effectually destroyed. Boiled water will not cause enteric fever or cholera, the two chief water-borne diseases.