248 The Rise of the Modern [lect. 



" If we augment or dimmish in any atmosphere the quantity 

 ** of air eminently respirable, we augment or diminish the 

 " quantity of metal which can be calcined in it, and to a certain 

 " extent the time during which the animal can live in it." 



Thus at a single stroke as it were did this clear-sighted 

 inquirer solve the problem of oxidation, and almost, if not 

 quite, the problem of respiration. He brought our knowledge 

 of the latter process very nearly to its present condition. 



Yet he went still a step further. 



Three years later, in 1780, he and the great mathematician 

 Laplace published their celebrated memoir on heat. 



In this memoir these authors, after placing the theory of 

 the heat of oxidation and combustion on a sound basis, after 

 describing their methods for determining the heat given out 

 during chemical action, the results which they obtained, and 

 the conclusions to be derived from those results, applied their 

 new views to elucidate the cause of the natural heat of living 

 bodies. 



As we have seen, Haller left the problem of animal heat in 

 an unsettled condition. The chemical theories of its origin had 

 fallen somewhat into disrepute ; but the mechanical theory, 

 that it was due to the friction of the blood in its movements, 

 though favoured by Haller did not seem to him to be wholly 

 satisfactory. 



Black, besides discovering fixed air, had prepared the way 

 for the true theory of heat by pointing out the distinction 

 between latent and sensible heat, and had introduced the ideas 

 of capacity for heat and of specific heat. In 1777-9 Adah- 

 Crawford published a theory of heat, based on Black's views ; 

 a theory which, as he put it forward, seems vague and hypo- 

 thetical, but which at least has the merit of connecting animal 

 heat and respiration in a way which had not been done before. 

 His theory was as follows : 



Inspired air contains elementary fire, and meets in the lungs 

 with the inflammable principle present in the blood. The 

 elementary fire leaves the air of the lungs to join the blood, 

 the capacity for heat of which is increased. In the course of 

 the circulation the blood again becomes impregnated with the 

 inflammable principle by which the capacity of the blood for 



