424 



Dr. W. Marcet. 



[Feb. 10, 



This table shows, in my own case, a theoretical excess of 8'3 per 

 cent, and 8'9 per cent, of carbonic acid expired for the first and 

 second series at Courmayeur respectively over my second series at 

 Yvoire, assuming the temperatures to be the same at both stations. 

 In M. David's case, the theoretical excess of carbonic acid expired at 

 Courmayenr over that expired at Yvoire, for similar temperatures, was 

 found to be 7 '7 per cent, and 8'9 per cent, for the two Courmayenr 

 series respectively. It follows that, for equal mean temperatures, 

 even such a small difference of altitude as 2,715 feet would have 

 given in these experiments a mean increase of carbonic acid of 8' 6 

 per cent, in my case and 8'3 per cent, with M. David. M. 

 Mermod,* who experimented with equal mean temperatures at a 

 difference of altitude of 3,143 feet (from 142 metres to 1,100 metres) 

 obtained an increase of carbonic acid expired by 6*7 per cent, at his 

 highest station, which is very near to my result although slightly 

 lower. His experiments were all made fasting, between 7 and 8 

 o'clock in the morning, and it is not unlikely that his comparatively 

 low increase of carbonic acid at the highest station was due in some 

 measure to his being in want of food at the time, and unable to 

 generate exactly the increased amount of carbonic acid required to 

 make up for the altitude. 



It may be asked, why should the body make more carbonic 

 acid at increasing altitudes, and under similar atmospheric tempera- 

 tures ? The reply is, that the evaporation from the skin arid lungs 

 must be greater under a lower pressure and drier atmosphere, giving 

 rise to a production of cold. The air was exceedingly dry while we 

 were at Courmayenr ; on July 21st, at 12.30 p.m., the readings of dry 

 and wet bulb thermometers were 78° and 62° respectively, giving a 

 relative atmospheric humidity of 39 per cent. ; the same day, at 2.15 

 p.m., the readings were 78° and 59°, corresponding to 32 per cent, of 

 relative atmospheric humidity. Under such a dry air there must have 

 been a considerable evaporation from the skin and lungs, cooling the 

 body and thereby more than making up apparently for the absorption 

 of heat, the result being an excess of carbonic acid expired. 



The twelve experiments for each of us on the summit of the Col 

 cannot be viewed in the same light as the others, because although 

 the cold we were subjected to under such a low atmospheric pressure 

 was comparatively great, and, indeed, keenly felt, still we both gave 

 out less carbonic acid than at Courmayeur instead of more. This 

 circumstance will find presently a ready explanation ; in the meantime 

 I shall propose to divide the twelve experiments on the Col into two 

 groups of six for each of us respectively, one group corresponding to 

 the lowest and the other to the highest atmospheric temperatures. The 



* " Bulletin de la Soc. Yaudoise des Sc. Nat.," vol. xv, 1877. 



