The constituent parts of water when pure are, in volumes, two parts of hydrogen and one part of oxygen, and, by weight, one part of hydrogen and eight parts of oxygen. When pure, water is also transparent, odorless, tasteless, and colorless, except when seen in considerable depths. Now, while it appears to be an established fact that chemically pure waters are not best for drinking purposes, still there is a limit in the condition of impurity beyond which it is not safe to imbibe. Rain-water is almost always affected by atmospheric influences; spring and well-water very often become charged with mineral properties, and, finally, the waters of rivers, lakes, and ponds, as a rule, contain more or less vegetable and animal organisms. But what is the minimum of safety? Dr. Frankland furnishes the following conclusions as to what must be considered polluted water:

1. Every liquid that contains in suspension more than one part, by weight, of dry organic matter in 100,000 parts of the liquid; or one part by weight, of dry mineral matter in 100,000 parts of the liquid.

2. Every liquid containing in solution more than two parts, by weight, of organic carbon, or three parts of organic nitrogen, in 100,000 parts of the liquid.

3. Every liquid which, when placed in a porcelain dish to the depth of one inch, exhibits during daylight distinct color.

4. Every liquid which contains in solution more than two parts, by weight, of any metal, except lime, magnesia, potash and soda, in 100,000 parts of the liquid.

5. Every liquid which contains in suspension more than 5/10 parts metallic arsenic, by weight, in every 100,000 parts.

6. Every liquid which, after the addition of sulphuric acid, contains more than one part, by weight, of free chlorine in every 100,000 parts.

7. Every liquid which contains, by weight, more than one part of sulphur, in the state of sulphuretted hydrogen or of a soluble sulphuret, in every 100,000 parts.

8. Every liquid possessing an acidity greater than that produced by adding two parts, by weight, of hydrochloric acid to 1,000 parts of distilled water.

9. Every liquid possessing an alkalinity greater than that produced by adding one part, by weight, of caustic soda to 1,000 parts of distilled water. 10. Every liquid exhibiting on its surface any film of petroleum, or containing in suspension more than 5/10 parts, by weight, of such oil in every 100,000 parts.

It is most important, says Dr. Austin, that we should seek to avoid all waters tainted by organic matter, especially sewage. The presence of ammonia in any water offers valuable evidence of such contamination; since it is the measure of that portion of organic matter not decomposed, but in a state of or undergoing putrefaction. More than 5/10 of a grain of free ammonia per 1,000 gallons of water, or more than 9/10 of a grain of albuminoid ammonia per 1,000 gallons of water, for bodes danger to persons drinking the liquid, or beverages manufactured from it.