24 



Inland Revenue Department, bulletin 5, 



page 8, No. 16 



Inland Revenue Department, bulletin 5, 



page 8, No. 17 



Inland Revenue Department, bulletin 13, 



page 6, No. 6761 



Inland Revenue Department, bulletin 13, 



page 15, No. 43 



MTROi;KN I'El^ MllJ.IUN PAins, AS 



In order to guard against miscomprehension 1 must mention liere 

 that nitrates although fairly stable compounds, are not absolutely such ; 

 but may, under certain conditions, be again re.solved into ammonia or 

 nitrogen. 



10. I have already referred to the universal employment of com- 

 mon salt as a condiment and preservative ; a fact which accounts for its 

 presence in sewage, and makes a search for it in water analysis a very 

 important step in the examination. The readiness with whic h minute 

 traces of kitchen salt can be recognized will be evident to you from 

 this experiment. WHien nitrate of silver solution is added to this solu- 

 tion of chromate of potash, a few drops of this weak solution is sufficient 

 to produce a decidedly reddish tint, due to the bright red chromate of 

 silver formtd in the re-action, the particles being suspended through 

 the water in the tube. I will now repeat the ex[)eriment, taking the 

 precaution to add a very small amount of common salt to the chromate 

 solution, before adding the silver, drop by drop, for a very long time 

 without producing any red colour in the liquid ; in fact, no chromate of 

 silver will be permanently formed until enough silver has been added 

 to decompose the common salt present. On this principle is based a 

 method by which we can detect less than i part of salt in i million 

 parts of water. Wherever sewage is present chlorides will be found. 

 In the four wells whose nitrates indicated past sewage contamination, 

 the chlorine in chlorides was found to be 148, 134, 65 and 143 parts 

 per million respectively. A large number of good wells whose analyses 



