At the International Pharmaceutical Congress held in Brussels,[128] the following standards of purity for potable water were recommended:-- 1st. A water should be limpid, transparent, colourless, without smell, and free of matter in suspension. 2nd. It should be fresh, with a pleasant taste, and its temperature should not vary much, and certainly not be higher than 15°. 3rd. It should not contain noxious animal or vegetable matter, and especially none of these substances in a state of decomposition. 4th. It should not contain more than 6 to 10 mgrms. of organic matter per litre, expressed in terms of oxalic acid. It should not contain nitrogenous matter. 5th. The nitrogenous organic matter, oxidised with an alkaline solution of potassium permanganate, should not yield more than 0·01 part of albuminoid ammonia per 100,000. 6th. It should not assume a disagreeable smell after having been kept in an open or closed vessel. 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. 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:--