This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
1. An easy and quite reliable test for organic matter in water is this: Add about ten grains of pure granulated sugar to about five ounces of the water to be tested; the bottle should be completely filled, and the stopper tightly fitted, so as to exclude the air. Expose the water to daylight and a temperature of about seventy degrees Fahrenheit. If it contains much organic matter, an abundance of whitish specks will appear within a day or two, floating around in the liquid. Of course the more organic matter there is, the more marked the appearance. These little bodies are best observed by holding the bottle against something black, or by partly shading the farther side of it with the hand. After a while they "will group themselves together in bunches, and partly settle to the bottom of the bottle; at length, if the water is very bad, the odor of butyric acid (the smell of rancid butter) becomes perceptible.
A chemist of repute says: If germs of any living organism are present, the water containing some sugar in solution will, after being kept in a warm place for about twenty-four hours, become cloudy, and sometimes quite milky or opaque, owing to the rapid development of fungoid organisms, resulting from the growth of the germs in a suitable nutritiva medium. The test is a valuable one, but requires to be used with caution. It is well to remark, however, that some chemists believe that the growth of the fungoid organisms is dependent upon the presence of phosphates, rather than upon any organic impurities, and that it is possible the germs may be derived from the air, and not from the water itself. Those who have experimented on the subject cannot have failed to observe how very varied is the behavior of different waters when treated with sugar.
2. Recently Dr. Smith, of Manchester, has pointed out that gelatine is most valuable in detecting organic vitality in waters. About 2 1/2 per cent, of gelatine, well heated in a little water, is mixed with the water to be tested, and the mixture forms a transparent mass, which is not movable like the water itself. When soluble or unobserved matter develops from the organic matter of the waters, and makes itself visible in a solid and insoluble form, it does not fall to the bottom, but each active point shows around it the sphere of its activity, and that sphere is observed and remains long. The gelatine preserves the whole action, so far as the more striking results are concerned, and keeps a record, for a time, both of the quality and intensity of life in the liquid. Dr. Smith speaks of the more striking effects, which are clear and abundant, every little centre of life making itself apparent to the eye, and sometimes expanding its influence to reach both sides of the tube. It seems to him now essential that all chemical examination of water should be supplemented by an inquiry into the comparative activity of the living organisms. If the water is pure, the gelatine cylinder remains long unaltered; but if it is impure from the presence of organisms, the gelatine round these becomes liquefied and globular, the organisms remaining solid at the bottom of the spheres. Dr. Angus Smith has prepared photographs of test-tubes of water which had been thickened by a solution of the purest fish-gelatine, and then exposed to the action of light. When the water was pure it remained translucent, but when bad, bubbles were rapidly formed, and the bacteria which appeared to be in the water began to act on the gelatine, breaking it up and rendering it soluble. A rapid movement of gas was observable. When the bubbles or balls appeared to be spherical, they indicated aggregations of bacteria. This change took place quickly, almost in twenty-four hours. But the test was only applicable where infusoria or fungi were present. For instance, peaty water in which there were no animalculae or bacteria would stand without breaking up the gelatine. To change the gelatine, organisms must be present. Organic matter that is not putrescent or infective will not do it.
The microscopic examination of water is of greatest importance and should always be combined with the chemical analysis.