OF NATURAL HISTORY. 105 



dies, varies according to their nature or texture ; that this fire, when 

 in a latent or quiefcent flate, is termed abfolute heat ; that, when 

 fubftances of different textures have a given quantity of heat thrown 

 into them, their temperature will be difcovered to be different by the 

 thermometer; for the fame quantity of heat which raifes one body 

 to a certain degree, will raife another to a greater or a lefs ; and this 

 different difpofaion of bodies is called their capacity of containing- 

 abfolute heat. 



Do£lor Crawford next endeavours to prove by experiments, that, 

 when phlogifton is added to any body, its capacity of containing 

 abfolute heat is diminifhed; and that, when phlogifton is abftrafted 

 from the fame body, its capacity of receiving abfolute heat is aug- 

 mented. Hence he infers, that heat and phlogifton feem to confti- 

 tute two oppofite principles in nature By the aftion of heat upon 

 bodies, the force of their attradion to phlogifton is diminiilied ; and, 

 by the a£tion of phlogifton, a part of the abfolute heat, which ex- 

 ifts in every fubftance as an element, is expelled. ' Hence,' fays 

 the Doiftor, ' animal heat feems to depend upon a procefs fimilar to 



* a chemical eledive attraction. The air is received into the lungs, 

 ' containing a great quantity of abfolute heat. The blood is re- 

 ' turned from the extremities, highly impregnated with phlogifton. 



* The attradlion of the air to the phlogifton is greater than that of 



* the blood. This principle will therefore leave the blood to com- 

 ' bine with the air. By the addition of the phlogifton, the air is 

 ' obliged to depofit a part of its abfolute heat ; and, as the capacity 



* of the blood is, at the fame moment, increafed by the feparatioa 



* of the phlogifton, it will inftantly unite with that portion of heat 

 ' which had been detached from the air, 



' We learn from Dodor Prieftley's experiments with refped: ta- 

 ^ refpiration, that arterial blood has a ftrong attraftion to phlogifton^ 



