-.SSj WATER ANALYSIS 53 



more than 0-35 part of nitrogen as nitrates per 100,000 

 parts, unless there is some satisfactory explanation. This 

 amount equals about one grain per gallon when expressed 

 as N 2 O 5 , or 1-5 parts per 100,000 when expressed as NO 3 . 



ICE. 



Ice is frozen water, and it is not usually purer in 

 content than the water from which it is derived. What- 

 ever may be frozen out of the water is usually mineral 

 matter, such as salt ; suspended matter is likely to be 

 enclosed. Microbic content, when composed of the com- 

 mon sewage organisms, is little affected by the temperature 

 of freezing, for the most part only being rendered torpid. 

 As far as possible, therefore, ice should only be used when 

 made from pure water, and by a process in which it is not 

 subject to risk of serious contamination. The analysis of 

 ice proceeds on the same methods as for water, the ice 

 being first melted. 



MINERAL WATERS AND AERATED WATERS. 



These are examined on the same principles. In the 

 case of artificial waters, the spring or supply from which 

 they are made should also be examined. In such also a 

 search should be made for poisonous metals, such as lead 

 and antimony, iron, copper, zinc, and even arsenic. 



In natural mineral waters the same careful examination 

 should be made. In these the mineral content is often 

 considerable, and a thorough analysis of the different 

 metals present is very important. The temperature and 

 the amount of carbonic acid gas are also noted. Nowadays 

 the presence of metals of the radium group has acquired 

 a new significance, and their occurrence is specially noted. 



INTERPRETATION OF THE RESULTS OF A 

 WATER ANALYSIS. 



This must not be based on any one item, but on a 

 careful consideration of the following points : 



1. Local inspection for any source of possible pollution. 



2. Bacteriological examination made as soon after collec- 

 tion as possible. 



