418 EESPIEATIOX. 



The indirect method was first employed by Boussingault, 

 but was particularly directed to the exhalation of carbonic 

 acid. This observer experimented upon large animals, such 

 as the horse or cow, in the following way : Having first care- 

 fully regulated the diet, so that there was no change in weight 

 during the experiments, he carefully weighed all that was 

 introduced as food and drink, and all that was discharged 

 as urine and feces. The excess in the quantity introduced, 

 over that discharged in the way above mentioned, represents, 

 necessarily, the amount lost by the skin and ]ungs. By a 

 quantitative comparison of the elementary constituents of the 

 food and excrements, tolerably accurate results were arrived 

 at ; though it must be admitted that this method would be 

 considered of little value, did the results not correspond pretty 

 closely with those obtained by direct analysis. 1 



Estimates of the absolute quantities of oxygen consumed, 

 or of carbonic acid produced, which are based on analyses of 

 the inspired and expired air, calculations from the aver- 

 age quantity of air changed with each respiratory act, and 

 the average number of respirations per minute, are by no 

 means as reliable as analyses showing the actual changes 

 in the air, like those of Regnault and Reiset, provided the 

 physiological conditions be fulfilled. When there is so much 

 multiplication and calculation, a very slight and perhaps 

 unavoidable inaccuracy in the quantities consumed or pro 

 duced in a single respiration will make an immense error 

 in the estimate for a day, or even an hour. 



Bearing all these sources of error in mind, from the ex- 

 periments of Yalentin and Brunner, Dumas, and others, a suf- 

 ficiently accurate approximation of the proportion of oxygen 

 consumed by the human subject may be formed. The air, 



human subject on a small scale in 1843, by Scharling, but there was no arrange- 

 ment for estimating the quantity of oxygen furnished (MILNE-EDWARDS, Physi- 

 ologic, tome ii., p. 498, note.) 



1 BOUSSINGAULT, Memoir -es de Chimie Agricolc et de Physiologic, Paris, 1854, 

 pp. 1-12. 



