OF WATER FOR ORGANIC MATTER. 11 



" A test like this depends partly on the eye and partly on 

 the delicacy of the test-paper. The same result will be 

 attained if the same eye and tests are regularly used. 



" The method of testing with paper is not so refined as 

 the use of a larger amount of water, say i ooo grains ; with 

 this amount, the presence of i of NO^ in 3 J millions of 

 water may be detected on adding starch and iodide of 

 potassium ; 3 drops of sulphuric acid are also added to the 

 1000 grains, i.e. 3 grains by measure. If no nitrous acid 

 is present, no blue colour will be seen with this amount. 

 If nitrous acid is present, the colour will begin in a few 

 seconds. Some may prefer one way, and some the other.^^ 



The methods given of estimating the amount of nitrous 

 acid are minimetric, proceeding by dilution instead of con- 

 centration. The value with gases is better known than 

 in liquids; but it is believed that it will be sufficiently 

 exact with the latter in cases where pure scientific accu- 

 racy is not attainable and not necessary, and where it is 

 important to save time, labour, expense, and patience. 



Suppose we find that a specimen of water contains 

 O'ooi gramme of nitrous acid in 1000 grms., or in the 

 quantity used, we find by calculation that this is equal to 

 0*000421 of oxygen, or, as the Table shows, to 0*842 of 

 the solution of chameleon used. 



Now suppose 1000 of the water decolorize 5 of chame- 

 leon, we must subtract from 5 the amount which would 

 be due to the NOj, 5-0— o-842 = 4-i58, which then is the 

 amount in c.c. of chameleon solution decomposed by the 

 organic matter, i of NO3 requires 0*42 1 of O to become 

 NO5. 



O'OOI of nitrous acid = 0*000421 of oxygen, 



or 0*001684 solid chameleon, 

 or 0*842 of the solution of chame- 

 leon of *2 in 1000. 



It will be seen that if we find the amount of O to which 



