THE MECHANISTIC CONCEPTION OF LIFE 157 



to be such a mechanical filtering apparatus as he postulated. It 

 is true that he could not see the pores, but neither can we even 

 with the aid of the microscope. 



In short, Descartes assumed that his pure mechanism was 

 operative beyond the limits of actual observation (and, as we 

 shall see, that is also assumed by us in respect of our chemical 

 and physical mechanism). 



The Influence of Chemical and Physical Investigations. — 



Now, apart from the influence of chemical discovery, it is not 

 easy to see how the Cartesian physiology need have become 

 modified. But the great advances made by chemistry towards 

 the end of the eighteenth century changed the point of view 

 entirely. The true theory of combustion, as it developed in the 

 hands of Priestley, Black, and Lavoisier, was very revolutionary 

 in its effect on the conception of the nature of vital activity. 

 • It was shown that when a piece of metal was strongly heated in 

 the air a calx (that is, an oxide) was formed, and that when 

 carbonaceous material was also made to glow in air some part 

 of the latter disappeared, and " fixed air " (that is, carbonic acid 

 gas) came into existence. Now the animal body, when dried, 

 ^was found to consist largely of carbonaceous material, and it was 

 discovered that when air was taken into the lungs some part of 

 it disappeared and was replaced by fixed air, just as in the case 

 ''of the combustion of carbon outside the body. The inference 

 was soon made that in respiration there was an actual process of 

 combustion, and that this occurred in the lungs, or blood, or 

 tissues, and was the source of animal heat. There was not, 

 therefore, an innate heat of the heart, and the latter became 

 f regarded solely as the propulsive organ of the circulation. With 

 these discoveries the Galenic physiology became obsolete. 



The modern idea of energy came later, and developed naturally 

 from the applications of the motive force of steam that were 

 made by Watt and the engineers, and the theory of the perfect 

 steam engine that was worked out by the French physicist, 

 Carnot. This great man showed that an engine did work by 

 taking heat from a source, and giving it up to a condenser, and 

 that there was a transformation of heat into mechanical energy. 

 His investigation became the foundation of our modern science 

 of thermo-dynamics, which, in its turn, became enormously 

 fruitful in the treatment of chemical problems. Later on 

 E-umford pointed out that mechanical friction generated heat 



