Until within a few years it was customary to judge of the purity of a water by a chemical examination. The quantity of organic matter which a water contained, the amount of free ammonia therein, the amount of so-called albuminoid ammonia which could be developed, the presence of chlorin not coming from the natural element of common salt, the amount of oxygen, which organic matter therein contained, consumed, were all regarded - and justly so - as marks of purity or impurity. There is no desire in this case to throw any doubt upon the value of these sanitary analyses. Such examinations are of the utmost importance. Since, however, the evil effects produced by water are rarely due to any inorganic substance, except in the case of mineral waters, nor to mere organic matter it is evident that a chemical examination is not sufficient to judge of the purity of the supply. In the present state of our knowledge we may say safely that a bacteriological examination is more important from a sanitary point of view than a chemical. The only doubt which attends a bacteriological examination is that as a rule the pathogenic germs which are in water are so few in number that they may escape detection. In all cases a very small quantity of the water is used for examination. This quantity is usually diluted necessarily in order that in seeding a sterilized medium therewith the number of bacteria which grow therein are not too crowded to be counted; hence, a very small portion even of the small portion originally used actually finds its way to the media used for growth. Hence in a water, say, contain-10,000 bacteria to the cubic centimeter and only 10 typhoid germs it is quite possible that in seeding the growth media not one of the 10 germs will find a lodgment. In order to avoid this in all cases of suspicion large numbers of bacterial counts and cultures must be made. In this way it is quite likely that the deadly germ for which the search is made will not long escape detection. Determination of the Pollution of Water.

Reference has already been made to the sanitary analysis of water by chemical means and to the sanitary analysis by means of bacteriological investigations. The question therefore arises as to how the pollution of a water may be accurately determined, and also can any conclusion be drawn as to the character of the pollution. In the cases of greatest pollution, that is where the water carries visible fragments of detritus of different kinds, a microscopic examination will usually reveal the character of the contamination. The various kinds of debris may be identified by structural or other features and thus easily traced to their source. One of the most common contaminations of running water is particles of silt. These are of greater or less size, according to the force of the current and the time which has elapsed since the fragments of soil have been detached. The examination of the water will reveal the character of the silt, especially where the particles are still large enough to have definite physical characteristics. The silt, however, may exist in a state of such fine sub-division as to remain in suspension for many days or even indefinitely. Such silt has no physical properties by means of which it can be identified. Fragments of food or excreta or particles of paper or other matters will indicate at once gross pollution with sewage or refuse from the dwellings. Fragments of wood pulp will indicate contamination from a paper factory. Discoloration due to tannin and particles of tan bark or fragments of hair or leather will indicate contamination from a tannery, and so on through the list of possible contaminations. When the character of the gross contamination is not sufficient to determine the source of pollution recourse must be had to sanitary, chemical and bacteriological analyses. Often the ordinary chemical analysis will reveal the source of pollution. This is the case wherever soluble matters are poured into the water supply from any source whatever and which are of a nature to enable the chemist to determine their character. Thus, colorless solutions, such as salts of sulphite or sulphate of soda or other soluble salts can be easily identified by analytical means.

If the matters producing the contamination are colored a particular tint or at least some shade thereof borne by the original matters will reveal the character of the pollution. Peaty and swamp waters are often revealed also by the color, which is usually caused by humus substances in solution or suspension. The character of the bacterial flora will also be an indication of the possible sources of pollution. Not only the nature of the substance found, but also the amount in which it occurs, has to be taken into consideration. This is not true of pathogenic bacteria, which if present even in very small numbers indicate a water unfit for consumption. Bacteriologists regard particularly as an indication of dangerous pollution the presence of bacillus coli and streptococci and many others which can be found described in standard works on the bacteriology of water.