60 DR. R. ANGUS SMITH ON THE EXAMINATION 



by several unwholesome gases, rapidly or instantaneously. 

 It is decomposed by fresh organic matter, and especially 

 organized matter, less rapidly. The putrid portion may 

 be estimated readily ; the latter is more uncertain. 



" In using this test, it is well to take not less than 5000 

 grains of the Avater, or still better 1000 grammes, i.e. one 

 litre j but if a small specimen only can be obtained, then 

 1000 grains may be used ; and it is well always to give the 

 figures for 1000 for uniformity, or (by multiplying by 70) 

 to change them into the amount per gallon. Grains or 

 grammes may be used indifi'erently, the solution being 

 made to contain simply so much in 1000. 



" A convenient plan is to use 2 grains of the pure crystal- 

 lized chameleon to 1000 of water. This, of course, is the 

 same as 2 grammes to a litre of water. 



" The chameleon is poured conveniently from a Mohr's 

 burette, or better still, when the water is very pure, from 

 a dropping-tube in which 1 c. c. (cubic centimetre) is 6 to 

 8 inches long. Add a drop of the chameleon, and wait till 

 the colour disappears; add another, and so on till the 

 colour remains permanent. The organic matter which 

 decomposes the chameleon in a minute or two must 

 be noted carefully; but generally there is a greater quan- 

 tity which decomposes very slowly; the result obtained 

 from the latter is, I believe, of less value. By decom- 

 posing in a minute or two, it is meant that when a few 

 drops are added, producing a slight colour, this undergoes 

 a change in a minute or two. But generally considerable 

 permanency is obtained in ten or fifteen minutes; then 

 the slow decomposition begins, of quite another quality 

 of organic matter, requiring hours or even days. This 

 matter must be estimated either by the weighing process 

 described, or by the further action of the chameleon, to 

 be described. The amount decomposed instantly is a true 

 measure of the putridity, it is believed. If there is very 



