Professor Mallet, who has devoted much attention to the investigation of potable waters, and whose opinion on this subject is entitled to the highest consideration, arrived at the following conclusions concerning the more vital points at issue in the determination of the hygienic character of water:—
“1. It is not possible to decide absolutely upon the wholesomeness or unwholesomeness of a drinking water by the mere use of any of the processes examined for the estimation of organic matter or its constituents.
“2. I would even go further, and say that in judging the sanitary character of the water, not only must such processes be used in conjunction with the investigation of other evidence of a more general sort, as to the source and history of the water, but should even be deemed of secondary importance in weighing the reasons for accepting or rejecting a water not manifestly unfit for drinking on other grounds.
“3. There are no sound grounds on which to establish such general ‘standards of purity’ as have been proposed, looking to exact amounts of ‘organic carbon’ or ‘nitrogen,’ ‘albuminoid-ammonia,’ ‘oxygen of permanganate consumed,’ etc., as permissible or not.
“4. Two entirely legitimate directions seem to be open for the useful examination by chemical means of the organic constituents of drinking water, namely; first, the detection of very gross pollution, * * * * and, secondly, the periodical examination of a water supply, as of a great city, in order that the normal or usual character of the water having been previously ascertained, any suspicious changes, which from time to time may occur, shall be promptly detected and their cause investigated.”
The microscopic and biological investigations of water are useful adjuncts to the chemical examination. The former is made by allowing a litre or more of the sample to remain at rest for several hours, collecting the deposit formed and inspecting it by means of the microscope, using low magnifying power at first. It will be found advantageous to stain portions of the sediment obtained with aniline violet, which, by a sort of predilection, attaches itself to particular forms of vegetable and animal life, thereby rendering them more distinct. The matters most usually observed in the microscopic examination of the deposit are:—
1st. Numerous lifeless substances, such as mineral matters, vegetable debris, muscular and cellular tissues, hairs, hemp, wool, cotton, silk, starch cells, insect remains, and pollen grains.
2nd. Living vegetable forms, such as confervæ, various algæ, oscillatoria, desmids, diatoms, and bacteria.
3rd. Living animal forms, including many varieties of infusoria and animalcula. Of the latter, those known as “saprophytes” are regarded as specially indicating the presence of sewage contamination.
Certain varieties of bacteria have been found associated with some forms of disease, and particular attention has been bestowed upon the study of these germs. The biological examination of water consists of pathological experiments on living animals, made by injecting a solution of the water-residue beneath the skins of rabbits, etc., and of experiments made by inoculating culture gelatine with the water. Of the latter methods of examination, that originally suggested by Dr. Koch, of Berlin, and described by Dr. Percy F. Frankland,[130] is well worthy of mention. In this process, the lower forms of life are cultivated in a solid medium, by means of which the growth of each colony is localised and rendered suitable for microscopic inspection.