7th. It should not contain white algæ, nor numerous infusoria, bacteria, etc.
8th. It must hold air in solution, which should contain a larger proportion of oxygen than ordinary air.
9th. It should not contain, per litre, more than:—
| 0·5 gramme | mineral salts. |
| ·060 „ | sulphuric anhydride. |
| ·008 „ | chlorine. |
| ·002 „ | nitric anhydride. |
| ·200 „ | alkaline earths. |
| ·030 „ | silica. |
| ·003 „ | iron. |
In the Municipal Laboratory of Paris, the following standards for potable waters are employed. One litre must not contain more than:—
| 0·5 to 0·6 gramme | total mineral residue. |
| 0·25 „ „ | calcium sulphate. |
| 0·015 „ „ | chlorine. |
| 0·005 „ „ | organic matter (calculated as oxalic acid). |
| 0·001 „ „ | albuminoid ammonia. |
| 0·001 „ „ | metals precipitated by sulphuretted hydrogen. |
| 0·003 „ „ | iron. |
| No sulphuretted hydrogen. |
100 c.c. should contain 3·25 c.c. of gas, 10 per cent. of which should be carbonic acid and 33 1⁄3 per cent. oxygen.
Professor J. W. Mallet[129] suggests the idea, that the noxious character of potable waters containing nitrates and nitrites, with but small proportions of organic matter, may be due to the presence of a special nitrifying ferment belonging to the lower organisms, which are capable of propagating disease.
In regard to the degree of importance that should attach to definite and arbitrary standards of purity, it appears to be accepted that, although the data afforded as the result of chemical tests are often of value in discriminating between pure and impure waters, but little reliance should be placed upon such criteria alone.