ix] Doctrines of Respiration. 249 



heat is diminished. It accordingly gives up heat to the tissues. 

 Thus in the lungs the blood discharges inflammable principle 

 and absorbs heat, in the system it imbibes this principle and 

 emits heat. 



It must be remembered that Black and Crawford, and 

 indeed Lavoisier, regarded heat or caloric not, as we now do, as 

 a form of energy, but as a thing or substance which combined 

 with the thing heated, a something which was the physical 

 analogue of the chemical phlogiston. 



Very different from Crawford's loose hypothesis is Lavoisier's 

 clear and succinct statement of the results of his and Laplace's 

 experiments. Having ascertained the amount of heat given out 

 by the combustion of a given weight of carbon into what now 

 began to be called not aeriform calcic acid but carbonic acid gas, 

 so quickly did knowledge advance in these few pregnant years, 

 and having determined on the one hand how much carbonic 

 acid was given out by, that is to say how much carbon under- 

 went combustion in the body of an animal during a given time, 

 and on the other hand how much heat was given out by the 

 animal during the same time, the authors found on comparing 

 the results, that the heat given out by the animal was about 

 the same as that given out by a quantity of carbon oxidized 

 so as to produce the amount of carbonic acid gas expired by 

 the animal during the time. 



They thus felt justified in stating the following conclusion. 

 "Respiration is therefore a combustion, slow it is true, but 

 " otherwise perfectly similar to the combustion of charcoal. It 

 " takes place in the interior of the lung without giving rise to 

 " sensible light because the matter of the fire (the caloric) as 

 " soon as it is set free, is forthwith absorbed by the humidity 

 "of these organs. The heat developed by this combustion is 

 "communicated to the blood which is traversing the lungs, 

 "and from the lungs is distributed over the whole animal 

 " system." 



A few years later, in 1785, Lavoisier was led to recognize 

 that he had been in error in supposing that respiration was a 

 combustion of carbon only. In a memoir entitled " The Changes 

 undergone by Respired Air," he made a careful quantitative 

 estimation of the quantity of oxygen (or ' vital air,' as he still 



