36 THE FOUNDATIONS OF SCIENCE 



chemical atom, as the smallest reacting particle, was 

 made more definite, and distinguished from the con- 

 ception of the physical molecule, the smallest particle 

 which could exist in the free state. In this form, the 

 theory is based on definite experimental evidence, 

 and not on speculative philosophical views about the 

 nature of matter, as it was in the days of the Greeks. 

 Chemistry is the direct descendant of mediaeval 

 alchemy, which sought to transmute base metals 

 into gold, and to prepare an elixir vitce or essence of 

 life, but in its search discovered many less harmful 

 things. By the investigation of the properties of 

 gases, and by the introduction of Dalton's atomic 

 theory, chemistry was placed firm on its modern 

 footing, where its conceptions have influenced all 

 other branches of science, and have become the basis 

 of the immense superstructure of molecular physics. 



One of the earliest developments of molecular 

 physics was due to J. P. Joule (1818-1889), who from 

 1840 to 1850 revolutionised the theory of the nature 

 of heat. Boyle (1627-1691), Newton, and other 

 acute thinkers, had previously held the view that heat 

 was due to the agitation of the ultimate particles 

 of bodies; but, in the absence of the modern con- 

 ception of energy, such a theory gave no foundation 

 for the measurement of heat as a quantity constant 

 throughout a series of changes. Hence, as often 

 appears in the history of science, a theory which was 



