38 DR. R. ANGUS SMITH ON THE 



In order to find the amount of fresli air which must be 

 supphed to any dwelHng, he explains that the carbonic acid 

 of the expired air is about 4 per cent., or 40 per mille. The 

 mean in the air is 0*5, and that of a good air for a room 



is O'?. This ffives a difference of 0'2. Then -^=200. 



' *= 0-2 



If "we must keep up the freshness of the air, we must add 

 200 times the vohime of the air that is expired. If a man 

 breathes out 300 litres in an hour, there must be added 

 60jOOO litres of fresh air=2ii8'96 cubic feet. At the 

 same time it is shown how much more rapid the change of 

 air is when out of doors, where the air is consequently 

 pleasanter, proving that even the apparently excessive 

 amount obtained indoors was not all that was desired. 



Pettenkofer mentions that it has been found necessary 

 in some hospitals in Paris to use exactly this amount of 

 60,000 litres in order to prevent all smell. 



If it were desired to keep the air at 0'6 per mille, the 



calctdation would be ^^=400 times. We cannot do better 



O'l 



than adopt the figures for bad and good air given by Dr. 

 Eeid, or rather Professor Pettenkofer, as far as they will 

 suit circumstances. Here we must put the amount of 

 carbonic acid in the atmosphere at 0*4 at the most, in- 

 stead of 0*5 ; and it is exceedingly probable that in many 

 parts of England it will be found constantly less, as it cer- 

 tainly is frequently. This would make it possible to have 

 a dwelling-room at 0"6, instead of 0*7 . But practically we 

 are not always favoured with such ventilation, even in 

 England ; and Germany seems to be much worse, judging 

 from the analyses of Pettenkofer, &c. In some cases we 

 may, with Dr. Reid, allow more carbonic acid. Dr. Reid 

 was not exaggerating, although he was frequently blamed, 

 when he insisted on supplying 600 cubic feet per hour 

 for an individual ; and we see in the ' Journal of the 



