The Eighteenth Century 183 



I would not even say that the light which is said to have proceeded from 

 some human bodies, of a particular temperament, and especially on some 

 extraordinary occasions, may not have been of the electrical kind, that 

 is, produced independently of friction, or with less friction than would 

 have produced it in other persons; . . . The electric matter in passing 

 through non-conducting substances always emits light. ... It is prob- 

 able, therefore, that electric light comes from the electric matter itself; 

 and this being a modification of phlogiston, it is probable that all light 

 is a modification of phlogiston. . . . 



Priestley expressed his views on heat as follows: " It appears to 

 me that heat has no more proper connection with phlogiston than 

 it has with water, or any other constituent part of bodies . . . the 

 heated state of bodies may consist of a subtle vibratory motion of 

 their parts." The speculations continue with a defense of phlogiston, 

 which received his ardent support throughout his life, even after the 

 theory had been abandoned by many of his contemporaries. 



Priestley's study of Airs also included observations on electric 

 behavior in different gases. He found that no kind of gas would 

 conduct electricity but that a spark would pass and the color was 

 white in " fixed air " and red or purple in " inflammable air." How- 

 ever, a Mr. Walsh and Mr. DeLuc had boiled the mercury in a 

 double barometer and hence obtained a more perfect vacuum, so 

 that the " electric spark or shock would no more pass through it 

 than through a stick of solid glass." Priestley had previously found 

 that sparks " diminished common air and make it noxious, making 

 it deposit its fixed air exactly like any phlogistic process; from 

 whence I concluded that the electric matter either is or contains 

 phlogiston." It is evident that the word " phlogiston " had made 

 such a lasting impression on Priestley that he found it impossible to 

 renounce the doctrine. 



The names of four contemporary men, Joseph Black (1728-1799) , 

 Henry Cavendish (1731-1810), Karl Wilhelm Scheele (1742-1786), 

 and Antoine-Laurent Lavoisier (1743-1794) , are associated with 

 that of Priestley. They all had much to do with the early history 

 of oxygen and combustion, but none of thein carried out extensive 

 research on luminescence.^^ Each played an important part in sepa- 

 rating the different kinds of gases, one of the great advances of 

 eighteenth-century chemistry. They thus paved the way for studies 

 on the effect of various gases on luminescence, experiments which 

 were so important in the later history of the subject. 



^^ A fore-runner of these men, Stephen Hales (1677-1761), best known for his work 

 on Haemastaticks (1727) , and Vegetable staticks (1733) apparently overlooked the 

 phosphorescence of the sea and the light of wood and flesh. 



