296 Medicine. [CfiAP. IV. 



sation in the lungs. Dr. Crawford, in 1779^ 

 adopted this opinion, and supported it by experi- 

 ments. They both believed that all the changes 

 produced by respiration are performed in the lungs j 

 and their theory differs but little in reality from 

 that of Dr. Black. They supposed the oxygen 

 gas of the atmosphere to combine in the lungs 

 with the hydrogen and carbon emitted by the 

 blood ; that, during this combination, the oxygen 

 gas sets free; a great quantity of caloric; and that 

 this caloric is not only sufficient to maintain the 

 temperature of the body, but also to carry off the 

 new formed water in the state of vapour, as well as 

 the carbonic^^cid, and to raise considerably the 

 temperature of the air expired. According to the 

 opinion, therefore, of these philosophers, the whole 

 of the caloric which supports the heat of the body 

 is extricated in the lungs. But on this hypothesis 

 the question will arise, how it happens that the 

 heat of each individual is maintained nearly the 

 same in every part of his body.^ To explain this. 

 Dr. Crawford endeavoured to prove, by well de- 

 vised experiments, that the capacities for contain- 

 ing caloric in arterial and venous blood are nearly 

 as 11.5 to 10; that is to say, if it require a quan- 

 tity of caloric, represented by 1 1 .5, to heat a pound 

 of arterial blood from zero to 30'', it will only re- 

 quire a quantity as 10 to heat a pound of venous 

 blood from zero to 30 ^ 



On these experiments the following conclusions 

 were formed. Oxj^gen gas is decomposed in the 

 hings in consequence of the affmity of the carbon 

 and hydrogen of the blood for oxygen being 

 greater than that of oxygen for caloric, or of the 



