244 ANNUAL OF SCIENTIFIC DISCOVERY. 



kaline. Thus, the air of sewers is invariably so, as has been proved 

 by the experiments of Dr. Dundas Thomson, its alkalinity being ow- 

 ing partly to ammonia, partly to the sulphuret of ammonium, the 

 form assumed by the sulphur disengaged in the decomposition of feecal 

 matter. The air of stables and stable dwellings is strongly alkaline, 

 as every one in attendance on the sick poor in London well knows ; 

 and the air expired by men and animals, although at first probably 

 acid, rapidly becomes alkaline by putrefaction. The relation be- 

 tween putrefaction and the existence of ammonia in the air is there- 

 fore so close that the detection of this body may, under ordinary cir- 

 cumstances, be regarded as a proof of its existence. 



Bleaching Power of the Air. It is known to those who are com- 

 mercially engaged in air-bleaching, that the bleaching power of the 

 air varies very considerably, not only according to the season and 

 time of day, but irrespectively of such periods. It has not yet been 

 determined what is the relation between this reaction and the oxid- 

 izing power of the air, as exhibited in its power of decomposing 

 iodide of potassium. But the remarkable experiments of M. Hou- 

 zeau, made simultaneously in town and country, have shown that 

 country air bleaches much more rapidly than town air, and that in 

 this respect the difference is no less marked than in that of the well- 

 known ozone reaction, which has been so clearly shown to be de- 

 stroyed by urban contaminations. 



The pressure, temperature, and moisture of the air are but little 

 modified by habitation. The readings of the barometer and hygrom- 

 eter in town and country do not differ. There is, however, one 

 respect in which the temperature of great towns may be favorably 

 compared with that of the country, viz., in that of equability. 



Mr. Glaisher found that in the middle of London the night tem- 

 perature is much higher, and the day temperature considerably lower, 

 than in the country, and consequently that the range of daily tem- 

 perature is nearly twice as great in the country as in London, espe- 

 cially in clear weather, when the cloudless sky of the country con- 

 trasts with the smoky obscurity of town. This immunity from great 

 variations of temperature must tend to diminish, though probably it 

 does not at all counterbalance, the generally injurious effects of 

 town air on persons affected with chronic pulmonary disease. 



In the preceding paragraphs we have reviewed all the differences 

 which are discoverable either by physical or chemical means between 

 the atmosphere of towns and that of the country ; and we are in a 

 better position to determine in the light of physiology which of these 

 conditions is likely to exercise most influence on the health of man. 

 As regards the existence of an excess of carbonic acid, it is clearly 

 of no importance whatever, for in many large towns no such excess 

 is met with. Sulphurous and sulphuric acids, if they have any influ- 

 ence, must act as " colytics," i. e., as agents tending to arrest putre- 

 factive change. The absence of sunlight, on which "the more equable 

 temperature of towns depends, has unquestionably an unfavorable 

 influence, but one which is very limited. We are driven then to the 

 only difference which remains, viz., that which depends on the exist- 

 ence of oxidizable matter, as indicated by its power of reducing certain 

 metallic oxides. Br. and Foreign Med. Review. 



