WHAT IS ELECTRICITY? — HEYL 217 



It was the middle of the sixteenth century before the next answer 

 on record was given to the question : What is electricity ? This an- 

 swer came from Cardan,^ whose name is familiar to mathematicians 

 (perhaps more so than it deserves to be). Cardan was the originator 

 of the fluid theory of electricity which held the stage in one form or 

 another for over 3 centuries, and survives today in popular parlance 

 in the term " the electric fluid ", or, still more colloquially, " the 

 juice." Cardan passed from the spiritual to the material in his ex- 

 planation, which was that amber " has a fatty and glutinous humor 

 which, being emitted, the dry object desiring to absorb it is moved 

 towards its source, like fire to its pasture; and since the amber is 

 strongly rubbed, it draws the more because of its heat." '^ 



In this last sentence we see the influence of Cardan's profession. 

 He was, among other things, a physician, and was accustomed to 

 warm the cupping glass in drawing blood from his patients. The 

 laws of pneumatics were not yet understood at that time, and it was 

 generally supposed that the cupping glass acted because of its heat. 



The fact that this " fatty and glutinous humor " was intangible 

 and invisible seems to have caused Cardan no embarrassment. We 

 may perhaps view this the more charitably when we think of the 

 contradictory attributes that later scientists have found it conven- 

 ient to assign to the luminiferous ether. 



The year 1551, in which Cardan published this theory, may be 

 taken as marking the end of the first era, in which electricity was 

 regarded as a soul or spirit. Its begimiing goes back beyond re- 

 corded history. 



The concept of electricity as a material substance contained in 

 certain bodies known as electrics was strengthened by the experi- 

 ments of Gilbert (1600), who showed that many substances besides 

 amber were to be included in this class, but the full development of 

 the fluid theory of electricity did not come until the middle of the 

 eighteenth century. In the meantime, von Guericke (1672) had in- 

 vented his sulphur globe electrical machine, which made electrical 

 experimentation easy on a large scale. With the facilities thus 

 placed at his disposal he discovered electrical conduction and electro- 

 static repulsion, the latter destined to be a phenomenon of prime 

 importance in later speculation on the nature of electricity. 



In the eighteenth century development of the fluid theory two 

 names are prominent, those of Du Fay and Franklin, each typifying 

 a separate trend in theory. 



Du Fay's experiments (1733 and later) chronologically preceded 

 those of Franklin. His most important discovery was that glass 



"Cardan, De subtllitate, lib. 21, Paris, 1551. 

 ^Benjamin, Tark, op. cit., p. 248. 



