272 DR. R. ANGUS SMITH ON THE 



by which dust may be made to interfere, even although the 

 act of sweeping should not take place. Still the dust- 

 gradations themselves are worth measuring. 



The result seems to be that a piece of glass of a definite 

 size hung up in any place will receive deposits of ammonia 

 or substances containing ammonia in a short time ; and 

 by washing the ammonia off with pure water and testing 

 it with a Nessler solution, it may be seen whether there is 

 too much or not. It is the simplest test for ammonia yet 

 found. Its discoverer deserves great thanks. It must 

 not be forgotten that we may have ammonia in very dif- 

 ferent conditions: it may be pure; or it maybe connected 

 with organic matter. This mode of inquiry is better 

 suited as a negative test, to show that ammonia is absent, 

 than to show what is present. When ammonia is present, 

 there may be decomposing matter ; when absent, there is 

 not. I am hoping to make this a ready popular test for 

 air — a test for sewer-gases"^, for overcrowding, for clean- 

 liness of habitations and even of furniture, as well as for 

 smoke and all the sources of ammonia. Of course it must 

 be used with consideration, and the conclusions must not 

 be drawn by an ignorant person. 



How far it may be used as a test of climate is a matter 

 to be considered also. 



After the above experiments I made another series of 

 trials with air, Nesslerizing the washings at once, and not 

 after laborious distillings, as in former cases ; the results 

 are very valuable, showing that we obtain comparative 

 quantities equally in this way. 



The amount of ammonia obtained in this ready way 

 does not give exactly the same results as the more labo- 

 rious methods which I have used ; but it may be taken as 

 the most convenient. It must be observed that the amount 



* For sewage also to a larger extent than it has yet been userl. 



