210 



ken in at each inspiration, nor the portion of it that 

 is changed, nor the number of respirations made in 

 a minute, are yet finally agreed on ; and, indeed, 

 from various causes, they must be subject always to 

 much variation. Greater accuracy, also, in the ex- 

 periments of Crawford and Lavoisier, as to the com- 

 parative quantities of heat, produced by respiration 

 and by combustion, is desirable ; and in those of 

 Dr Menzies, the abstraction of heat by the air and 

 vapour expelled from the lungs, appears to be much 

 overrated. In their present state, therefore, these 

 inferences must be considered only as approximations 

 to the truth ; but the particulars on which they rest, 

 seem to embrace all the necessary considerations, if 

 in every instance they were accurately determined. 

 At any rate, it may, we think, be safely held, that in 

 the lungs of animals, sufficient air is decomposed to 

 furnish a large quantity of heat ; and the fact, that 

 the temperature of all animals is in proportion to 

 the relative capacity which their respiratory organs 

 bear to the body, and to the quantity of air which 

 they breathe in a given time, appears to justify the 

 conclusion, that animal temperature is derived chief- 

 ly, if riot entirely, from this source alone. 



195. But we have seen, that the oxygen gas of 

 the air is converted into carbonic acid by the human 

 skin (147.) and intestines, as well as by the lungs ; 

 and in many of the inferior animals, their superiori- 

 ty of temperature is sustained by the changes indu- 

 ced on the air through the agency of the skin alone. 

 As, therefore, the skin, by the same powers, changes 

 the air after the same manner as the Jungs, it is rea- 

 sonable to infer, that phenomena similar in kind. 



