The fact of extreme variations in the composition of water from the same source, points to the necessity of frequent examinations at various seasons, and under different conditions, to determine its true value for the intended purpose. As, however, changes of the water supply in manufactories are practically impossible, daily repeated water analysis impracticable, the application of methods become imminent by which the water is under all circumstances kept in the desired state of purity.

With few notable exceptions, soluble solid substances dissolve in a larger proportion at a higher than at a low temperature; the invariable rule for the solubility of gaseous substances in water is the reverse - that is to say, the lower the temperature of the water the more of a gas it is able to hold in solution under the same pressure. "For instance, under ordinary atmospheric pressure the co-efficient of solution of gases at different temperatures - an interesting study - is as follows, to wit:

At 32° Fahrenheit.

At 68°.

For oxygen

0.04114

0.02838

For carbonic acid

1.7967

0.9014

For ammonia

1049.6

654.

As tables of this kind are not fully comprehended by many not accustomed to them, it should be explained that while water at 32° can dissolve 411 parts of oxygen, or 1,796 parts of carbonic acid, or 10,496 parts of ammonia, it can dissolve but 284 parts of oxygen, or 901 parts of carbonic acid, or 6,540 parts of ammonia respectively, at a temperature of 68° Fahrenheit. Showing that a rise in temperature of only 36° Fahrenheit reduces the amount of gas which can be dissolved by water to nearly one half in the case of carbonic acid, and not much less in the case of the other gases.

It should be mentioned that, while the atmospheric air is composed of 20.96 parts by volume of oxygen, and 79.04 of nitrogen, the air held in solution by water through which air has been forced is found to consist of 34.91 parts by volume of oxygen, and 65.09 of nitrogen. Or, in other words, while in atmospheric air the oxygen forms but little over 1/5 that represented in the air dissolved by water is over 1/3, showing that the affinity of the water for oxygen is much greater than for nitrogen.

The great importance of this circumstance for the purification of water becomes evident from the fact, that to the oxidizing action of oxygen upon soluble albumenoid organic pollution in the water is mainly due the self-purification of the water of running streams, and indeed also the purifying action of charcoal. The pores of charcoal, especially animal or bone charcoal, hold oxygen in a highly condensed state, ready to be given off to other substances that come in intimate contact with it and have greater affinity for the oxygen. Many organic coloring and soluble albumenoid matters possess this affinity in a high degree, and are readily acted upon by fresh charcoal - that is to say, as long as its store of oxygen lasts. For it should be distinctly understood that the action of charcoal is by no means infinite, and too much is popularly expected of charcoal: to be a perfect purifier and also perfect percolator, acting simultaneously in both capacities.

The mineral suspended impurities are considered, physiologically, of no serious moment, except that the public naturally prefer a clear, bright water to a thick and turbid one. A careful filtration will remove suspended matter. The dissolved mineral impurities must be displaced or precipitated by chemical purification of the water or by boiling; the suggested remedies we will find later on.

The dissolved organic impurities in water are of two kinds - animal and vegetable. Of the latter we need say very little. They are for the most part, very difficult of removal, and, fortunately, are perfectly harmless.

The presence of the dissolved animal organic impurities are undoubtedly very objectionable, and of most serious import. These animal impurities are formed chiefly of carbon, hydrogen, nitrogen, and sulphur. When in solution in the water, these impurities undergo perpetual change and constant chemical re-arrangements, and the danger of drinking the water charged with them is specially great when these changes are taking place. The great chemical difference between animal and vegetable organic matter is the relatively large amount of nitrogen present in animal organic matter. In vegetable organic matters this element only occurs in comparatively small quantity. Substances containing nitrogen - the element of all others prone to change its relationship, and so to induce changes in the bodies containing it - constitute a class of chemical compounds called "ferments" - that is, bodies allied to yeast. And just as yeast sets the wort at work, resolving the sugar into alcohol and carbonic acid, so the nitrogenized organic matter of water in a state of perpetual alteration and movement, effects changes in the human body which result in disease.

Of the serious results of drinking water contaminated thus with animal organic impurity, numerous illustrations might be given. There can be no doubt, therefore, that the use of water containing animal organic impurities, which, as we have said, are themselves so liable to undergo change, and, when in this condition, to induce changes in other bodies with which they come into contact, is most dangerous to health, and may prove fatal to life.

But, it may be remarked, that when the surface wells, receiving, as they do, enormous quantities of animal contamination (sewage), and perhaps, as is often the case in a city, the drainage of churchyards, are tested by chemical analysis, they are very frequently found to contain very little actual organic matter. This is no doubt true, but, fortunately, the organic matter leaves behind it indisputable records of its previous existence, and, if we read these records aright, they warn us of the dangerous consequences that may any day arise from using such water. For when in surface-wells the chemist discovers alkaline and earthy nitrates, and a large quantity of common salt, these constituents, although they may be in themselves harmless, suggest to him the previous existence in the water of the filthiest impurities - such, for example, as the fluid matters discharged from the human body, and the percolations from cesspools and sewers. The decomposing matter in solution, by its slow passage through a considerable bed of earth, the soil exerting, as it is capable, its wonderful power of effecting the oxidation of the organic matter, becomes burnt up, its carbon being converted into carbonic acid, and its nitrogen into nitric acid. These constituents, it is to be specially noted, impart to the water an agreeable taste and a sparkling appearance, and so people like the water, and, because they like it, think it must be good. But things are not always what they seem. The agreeably deceptive properties of pleasant taste and good looks, tell how these wells are the gathering-place for surface-springs loaded with foul animal remains, and the washings from many fat churchyards. If, however, the continuance of this oxidation of the organic matter by the soil could be guaranteed, no harm. it is true, would come of it, but experience proves such guarantee is impossible. The salutary influence of the soil may fail by being worn out or overtaxed, so that, at any time, the putrid animal contamination may find its way into the well unchanged, thereby charging the water with the active agents of disease.