74 THE REALITIES OF MODERN SCIENCE 



for his miner's safety lamp, demonstrated by breathing 

 nitrous oxide, that is, " laughing gas," that it was not 

 the " principle of contagion 7 ' which the medical men of 

 his day considered it. 



The development of chemistry, as medicine, extended 

 only from the 15th to the 17th century. During the 

 17th and 18th centuries the influence of the alchemistic 

 theories was decreasing. With the discoveries of 

 Black, Priestley, Cavendish, Scheele and others in the 

 second half of the 18th century the mystical character 

 disappeared and it was possible for Lavoisier and Dalton 

 to lay the foundation of modern chemistry. 



The revolt against the doctrines of the medieval ages 

 was really started by Robert Boyle. In 1662 he pub- 

 lished his " Skeptical Chemist." In this he denied the 

 accepted theory and stated that all substances were 

 either elements, which could not be further decomposed, 

 or else compounds of two or more such elements. Com- 

 pounds, according to his hypothesis, were formed by 

 the coalescence of small particles of the elements 

 concerned. The corpuscles of any one kind he con- 

 sidered to have an affinity for the corpuscles of another 

 kind, and this affinity was the cause of the formation 

 of the compound corpuscles. The affinities postulated 

 by Boyle were not, however, the likes and dislikes and 

 other mental attributes sometimes used by his contem- 

 poraries or predecessors to explain chemical reactions. 



For centuries the theory of the four elements had 

 failed to account, even to its adherents, for the forma- 

 tion of the known products of chemical reaction. Ad- 

 ditional principles had, therefore, been accepted as 

 capable of modifying the nature of matter. Mercury 





