508 



Messrs. Carnelley and Haldane. 



given in the table of results (pp. 504 — 507). It will be seen that 

 the sewers which, we examined were all of considerable size, large 

 enough to be entered without great difficulty. Our data, therefore, 

 do not apply to small sewers and drains. 



In each sewer examined we estimated simultaneously the amounts 

 of carbonic acid, " organic matter," and micro-organisms, as in the 

 case of our observations referred to above on the air of houses and 

 schools. Analyses of outside air in the immediate vicinity of the- 

 sewers examined were made at as nearly as possible the same time. 

 In order to avoid as far as possible contaminations due to our own 

 presence we kept to leeward of the apparatus employed in collecting the 

 samples. The methods employed were that of Pettenkof er for carbonic 

 acid, Carnelley and Mackie's modification of the permanganate process 

 for organic matter,* and Hesse's method for micro-organisms. t The 

 results obtained are given in the preceding table. 



For the purpose of giving a general idea of the relative impurity 

 of sewer air we have taken the averages of analyses A and B of 

 sewer air and placed them alongside of the averages for outside air 

 at the same time, and for various classes of houses and schools, as 

 determined by us in the winter of 1885-86 and detailed in the paper 

 mentioned above. 



The above table shows (1) that the air of the sewers was much 

 better than one might have expected ; (2) that the carbonic acid was 

 about twice, and the organic matter rather over three times as great 

 as in outside air at the same time, whereas the number of micro- 

 organisms was less ; (3) that in reference to the quantity of the three 

 constituents named, the air of the sewers was in a very much better 

 condition than that of naturally ventilated schools, and that with the 

 notable exception of organic matter it had likewise the advantage of 

 mechanically ventilated schools ; (4) that the sewer air contained a 

 much smaller number of micro-organisms than any class of house. 

 The carbonic acid was rather greater than in the air of houses of four 

 rooms and upwards, but less than in two- and one-roomed houses. As 

 regards organic matter, however, the sewer air was only slightly better 

 than the air of one-roomed houses, and much worse than that of the 

 other classes of house. % These facts are brought out more clearly in 

 the following table, in which the average quantity in excess of outside 

 air of each constituent in sewer air is taken as unity. 



* < Koy. Soc. Proc.,' vol. 41, p. 238. 



f ' Mittheilungen aus dem k. Gesundheitsamte,' vol. 2, p. 182. 

 X The data for all the classes of houses refer to sleeping-rooms when occupied 

 during the night. 



