290 BIOLOGICAL AND SANITAEY MATTEBS. 



and 0-2 c.c. of — acid have been added ; calculate tlie number 



of c.c. used to the total volume (i.e., if 90 c.c. have been taken, 

 divide by 0-9), and subtract 0-2 c.c. Titrate in the same manner 



the volume of -— soda added, and the difference between the two 



results multiplied by 2-5 will give the total hardness as parts of 

 calcium carbonate per 100,000. 



The amount of soda solution added should be such that at 

 least half the quantity of acid is required to neutralise the filtrate. 



The permanent hardness is obtained by subtracting the tem- 

 porary hardness from the total hardness. 



Interpretation of Results of Water Analysis.— Good waters 

 contain, generally speaking : — 



Total solids, 2U to 40 parts j)er 100,000. 



Chlorine, . . . 1 to 2 ., 



Free ammonia, . not more than 0-001 



Albuminoid ammonia, ,, ,, 0-010 ., 



Nitric acid, . to 3 ., 



Nitrites, . . none. 



They absorb less than 0-1 part per 100,000 of oxygen, and are practically 

 free from phosphates. 



The total solids may be higher than the limits Jianied, in chalk 

 waters and in mineral waters. 



A high chlorine content may be due to beds of rock salt — e.g., 

 in waters from the new red sandstone — or to admixture with 

 salt derived from the sea (near the coast) ; it is, however, usually 

 due to sewage. 



Deep well waters often contain large amounts of free ammonia ; 

 and water which has passed through iron pipes may also contain 

 free ammonia and nitrites. 



A high albuminoid ammonia is usually very undesirable, 

 though not conclusive of pollution by sewage ; pools into which 

 dead leaves fall may give rise to high albuminoid ammonia. 



Nitric acid is a most reliable datum ; any amount above 

 .3 or 4 parts per 100,000 is certainly due to pollution. 



The presence of nitrites is always unfavourable, except when 

 the water has passed through iron pipes, and in chalk waters. 



The amount of oxygen absorbed does not give much infor- 

 mation as to whether a water is polluted with sewage ; high 

 figures are often due to vegetable matter. The proportion of 

 oxygen absorbed to albuminoid ammonia is often a useful datum. 

 Where vegetable contamination has taken place the oxygen 

 absorbed is ten times (or more) the albuminoid ammonia ; in 

 polluted waters it is usually less. 



