PHYSICAL SCIENCE 35 



vived the Dark Ages, and dominated later medieeval 

 thought. 



Galileo's experiments banished from mechanics the 

 conception of a body intrinsically light ; but, misled 

 by the phenomena of flame, chemists still repre- 

 sented burning as the loss of a substance, phlogiston, 

 which, since the balance showed a simultaneous 

 gain, must possess the Aristotelian property of a 

 negative weight. It was not till, towards the end 

 of the eighteenth century, the discovery and investi- 

 gation of several new gases had changed the point of 

 view, that burning was recognised as a combination 

 with oxygen, one of the gases of air, and phlogiston 

 vanished from the nomenclature of science. 



Chemical change was then seen to be concerned 

 with bodies all essentially of the same nature, and 

 a detailed study of chemical combinations showed 

 that a compound always contained the same definite 

 proportions of its elements; while, if two elements 

 formed more than one compound, simple relations 

 held between the proportions in the different com- 

 pounds in which the elements combine. 



In 1808 John Dalton (1766-1844) saw that these 

 relations were best explained by a revival of the old 

 atomic theory, the combining weights of the elements 

 giving a clue to the relative weights of their respec- 

 tive atoms. By the further investigation of the laws 

 of the combination of gases, the conception of the 



