242 ANNUAL OF SCIENTIFIC DISCOVERY. 



is to be supposed that whenever the acidity of the atmosphere is 

 marked, it will be mainly owing to the latter. The impregnation 

 of the rain with a mineral acid must be regarded as rather beneficial 

 to health than otherwise, as tending to retard the putrefaction of 

 animal matter on which it falls. 



Dr. Dundas Thomson appears to have been among the first to rec- 

 ognize the importance of organic matter as a constituent of the air 

 of towns, and to express the conviction that the gaseous products 

 evolved during putrefaction are not the main sources of danger. 

 Proceeding on this idea, he subjected a large quantity of atmospheric 

 air to chemical investigation, " with a view of condensing any vapor, 

 or detaining solid particles, which might be disseminated." The 

 result was entirely negative. Further inquiries of the same kind 

 were made, under the sanction of the Board of Health, in 1854, the 

 air being passed, as has been already mentioned, through distilled 

 water, the result invariably being that fungi made their appearance 

 in the water, and in a short time, by their rapid growth, pervaded 

 the whole of it, so as to be evident to the unassisted eye. It was 

 also found that by passing the air through sulphuric acid in the same 

 manner, the acid became soon dark colored, in consequence of the 

 charring of the organic matter introduced into it. Dr. A. Smith has 

 worked out the idea much more completely. He has preferred a 

 chemical to a microscopical test for the detection of the suspended 

 organic matter. It consists, as most of our readers may be aware, 

 in passing the air through a very dilute solution of permanganate of 

 potash, the strength of which is determined by ascertaining how 

 much is required to decompose a solution of a weighed quantity of 

 oxalic acid, or of uncrystallized sugar. This test obviously indicates 

 not the quantity of organic matter, but the quantity of oxidizable 

 matter, in the atmosphere, and hence it is only valuable in so far as 

 we may assume that the atmosphere contains no reducing agent. 

 Thus, in the presence of sulphurous acid, it would be of little value 

 were it not that that agent exists, even in the town air, in exceed- 

 ingly small quantity. Many of Dr. Smith's results are of such a na- 

 ture as to be beyond the possible limits of this source of error. It was 

 found that the same quantity of the solution of permanganate which 

 was decolorized by one bottle of air obtained in a close court in Man- 

 chester, required twenty-two bottles to decolorize it on the hills in the 

 neighborhood. Assuming that sugar and the organic matter of the 

 air are decomposed by the same amount of manganate, " a supposition 

 which cannot be perfectly true, but which, from the minuteness of the 

 amounts, leaves no room for a great error," Dr. Smith concludes 

 that whereas, on the high grounds north of Manchester, there existed 

 but one grain of organic matter in 200,000 cubic inches, in close 

 places in the town there was a grain in 8000 cubic inches. From 

 his most recent observations he concludes that we have " in different 

 air breathed by people in the same country a substance the amount 

 of which in one case is twenty-two times greater than in the other, 

 and in air breathed by people in the same town a difference which 

 is as nine to twenty-two." 



The whole importance of these investigations, regarded from the 

 point of view of preventive medicine, lies in their relation to the 



