OF WATER FOR ORGANIC MATTER. 85 



and common salt in the soil ; but these salts in all pro- 

 bability result from oxidized animal matter. Chlorides are 

 found in some districts in this country rather in excess 

 of the average from superficial causes. But notwith- 

 standing all these exceptions^ which appear for the moment 

 numerous, I must still consider that the test is one which 

 may be generally used. 



In England it is almost universally the case that the 

 presence of much chlorine in drainage-water indicates 

 drainage from animal matter; and no water containing 

 chlorides to a great extent ought to be used without careful 

 examination as to the source. One grain per gallon is too 

 much, and is, in many places, to be suspected of being 

 caused by impure drainage. Of course we must in this 

 case, as in all inquiries, be careful that no disturbing 

 causes intervene. In this country chlorides may be given 

 out from manufactories, in which are constantly made che- 

 mical experiments sufficiently large to interfere with our 

 accuracy, if we are not very careful. 



It may be supposed here that I am adding many quali- 

 fications ; but the same may be done in the case of nearly 

 all experiments. No experiment is of value unless it is 

 viewed on all sides to prevent the admission of errors ; and 

 I do not know that more care is required in this case than 

 in many others. In the case of chlorides we learn readily 

 the average amount in a district, either in the rain or the 

 drainage, and we detect the smallest increase. Any 

 amount of common salt above the average of the district 

 obtained in a well in a city or camp, or near habitations 

 of men or animals, is an almost certain proof of impure 

 drainage ; when the clue has been followed up, I have 

 foimd the origin in a sewer or some such spot, times 

 without number and for many years. The presence of 

 the sea or manufactories, or of disturbed strata with mi- 



