4. _Chlorine._--The importance attached to the estimation of chlorine in potable waters is derived from the fact that this element enters largely into the food of men and animals, and is thrown off in their excreta. This, naturally, contributes to the sewage contamination to which water is often exposed. Water, however, may take up a certain proportion of chlorides from the geological strata through which it passes, and it is of importance to bear this fact in mind in forming a conclusion as to the significance of the results afforded by this determination. It is, likewise, to be remembered that vegetable organic pollution would escape detection were the quantity of chlorine contained alone taken into consideration. The determination is conveniently made as follows:--50 c.c. of the water are introduced into a beaker, a drop or two of a concentrated and neutral solution of potassium chromate added, and then a standard solution of silver nitrate very cautiously added from a burette, drop by drop, until a faint but permanent red tint is produced. If the silver solution is prepared by dissolving 2·394 grammes of the nitrate in 1 litre of distilled water, the number of c.c. required to cause the reddish coloration directly indicates the parts of chlorine present in 100,000 parts of the water examined. According to Frankland, 100,000 parts of water from various sources contain the following proportions of chlorine:-- Rainwater 0·22 Upland surface water 1·13 Springs 2·49 Deep wells 5·11 Watts’ ‘Dictionary of Chemistry’ quotes the proportions below:-- Thames, at Kew 1·21 Thames, at London Bridge 6·36 Loch Katrine 0·56 Rhine, at Basle 0·15 Rhine, at Bonn 1·45 Lake of Geneva 0·67 Elbe, near Hamburg 3·94 Loire, at Orleans 0·29 The amount of chlorine contained in sewage is stated to range from 6·5 to 21·5 parts, the average being 11·54 parts.[123] It is generally considered that a proportion in excess of 5 parts in 100,000 parts of a drinking water, which is not liable to be affected by mineral admixture, is to be ascribed to organic contamination.