TEMPERATURE AND SEASON. 175 



erated, or naturally increased in frequency, as it is after feed- 

 ing, active exercise, &c. By diminishing the frequency, and 

 increasing the depth of respiration, the percentage proportion 

 of carbonic acid in the expired air is diminished ; being in the 

 deepest respiration as much as 1.97 per cent, less than in ordi- 

 nary breathing. But for this proportionate diminution also, 

 there is a full compensation in the greater total volume of 

 air which is thus breathed. Finally, the last half of a volume 

 of expired air contains more carbonic acid than the half first 

 expired ; a circumstance which is explained by the one por- 

 tion of air coming from the remote part of the lungs, where it 

 has been in more immediate and prolonged contact with the 

 blood than the other has, which comes chiefly from the larger 

 bronchial tubes. 



c. Influence of External Temperature. The observations 

 made by Vierordt at various temperatures between 38 F. and 

 75 F. show, for warm-blooded animals, that within this range, 

 every rise equal to 10 F. causes a diminution of about two 

 cubic inches in the quantity of carbonic acid exhaled per 

 minute. Letellier, from experiments performed on animals 

 at much higher and lower temperatures than the above, also 

 found that the higher the temperature of the respired air (as 

 far as 104 F.), the less is the amount of carbonic acid exhaled 

 into it, whilst the nearer it approaches zero the more does the 

 carbonic acid increase. The greatest quantity exhaled at the 

 lower temperatures he found to be about twice as much as the 

 smallest exhaled at the higher temperatures. 



d. Season of the Year. Dr. Edward Smith has shown that 

 the season of the year, independently of temperature, also ma- 

 terially influences the respiratory phenomena ; for with the 

 same temperature, at different seasons, there is a great diversity 

 in the amount of carbonic acid expired. According to his 

 observations, spring is the season of the greatest, and autumn 

 of the least activity of the respiratory and other functions. 



e. Purity of the Respired Air. The average quantity of car- 

 bonic acid given out by the lungs constitutes about 4.48 per 

 cent, of the expired air ; but if the air which is breathed be 

 previously impregnated with carbonic acid (as is the case when 

 the same air is frequently respired), then the quantity of car- 

 bonic acid exhaled becomes much less. This is shown by the 

 results of two experiments performed by Allen and Pepys. In 

 one, in which fresh air was taken in at each respiration, thirty- 

 two cubic inches of carbonic acid were exhaled in a minute; 

 whilst in the other, in which the same air was respired repeat- 

 edly, the quantity of carbonic acid emitted in the same time 

 was only 9.5 cubic inches. They found also that, however 



