300 



Dr. W. Marcet on 



[Apr. 11, 



The sign — before the figures, showing differences of temperature, 

 means that those temperatures are by so much lower than the mean 

 temperature at Yvoire ; while the sign + in the next column means 

 that the corresponding figures are by so much in excess of those show- 

 ing the amount of carbonic acid expired at Yvoire. An inspection of 

 this table shows that there is a marked excess of carbonic acid expired 

 at every one of the high stations over the amount expired at Yvoire ; 

 and, moreover, that this excess for the summit of the Breithorn is more 

 than twice that registered for the St. Theodule, just double that for 

 the Eiffel, and not quite twice that for the St. Bernard. 



It is difficult to conclude that this very great proportional excess 

 of carbonic acid exhaled at the Breithorn is due entirely to the cold air 

 met with at that station ; and there is, to say the least, a strong pre- 

 sumption of the existence of another cause to account for the forma- 

 tion of more carbonic acid in the body at increasing altitudes. A 

 further consideration of this point will lead to the conclusion that the 

 evaporation from the skin and lungs increasing as the atmospheric 

 pressure falls on rising above the sea, a certain degree of cold is 

 thereby produced. Its effect becomes added to the influence of the 

 fall of atmospheric temperature met with as a rule during the progress 

 of mountain ascents ; to be only resisted by the body by additional 

 combustion, attended with an increased production of carbonic acid. 

 In Mr. Mermod's experiments, the temperature of the air was the 

 same at his two stations, and the cold from increased evaporation 

 } ,ccounts apparently for the excess of carbonic acid expired at his 

 highest station. 



I have therefore come to the conclusion that there is an increase of 

 carbonic acid expired as a person rises above the sea on a mountain 

 excursion, and that this phenomenon is due to two causes — first, the fall 

 of the atmospheric temperature (this is shown as a fact), and secondly, 

 the cold produced by increased evaporation from the body, due to the 

 diminished pressure of the atmosphere, this last conclusion being drawn 

 as an inference. In short, more carbonic acid is formed in the body to 

 counterbalance the influence of cold from the two circumstances just 

 stated. If on ascending to a higher level we should find the same 

 atmospheric temperature as we left at the lower station, still an in- 

 creased amount of carbonic acid will be expected on account of the 

 cold due to the greater cutaneous and pulmonary evaporation. 



As to the volume of air breathed at different altitudes, it is remark- 

 able that, although under barometric pressures very different from 

 each other, and consequently with ever-changing bulks of air actually 

 inhaled, still when these volumes of air are reduced to that they 

 would occupy at the sea-side and at the freezing temperature, we find 



over, I took less food there than at the lower stations. These circumstances pro- 

 bably account for the unexpectedly low excess of carbonic acid exhaled at that place. 



