Tl02 THE PATH OF SCIENCE 



lowed by the detection of waves too short to pass through the 

 air and then by the proof that the x-rays (page 106) are very 

 short eleGtromagnetic waves of the same nature as light waves. 

 As we shall see later, the most recent work on the nature of 

 ,electrornag;Hetic waves has brought us back to the conception 

 that all wayes are associated with particles and that the long 

 .controy-ersy between the wave theory and the corpuscular 

 •theory .can ;b;e j.esolved to some extent in a compromise. 



Only t-^y<9 manifestations of the properties of electricity 

 were knawn ^p the ancients. They knew that magnetite ore 

 would attract aBcJ be attracted by iron and that amber when 

 rubbed would attract light particles, straw, paper, etc. In the 

 Middle Age^ it wa^:? found that a suspended piece of magnetite 

 would poi^t i0.c)ir!tji :and south, and the mariner's compass was 

 invented. At;the time when Galileo was working in Florence, 

 an English physician, William Gilbert, was carrying out ex- 

 periments on the magtiet and the attracting properties of sub- 

 stances ys^hic^h Ji:^d J^^n rubbed, and he showed that the be- 

 havior of the ,camTp^a<ss was due to the fact that the earth itself 

 was a great magnet. Gilbert also found that other substances 

 than amber=--glass^ ^stiifur, and resin— ^vould attract light par- 

 ticles after they had been rubbed. He wrote the first text- 

 book on electric-al ^scieriee, in which he discussed his experi- 

 ments. 



At the beginning of the eighteenth century, Stephen Gray 

 found that an electric charge, the existence of which ^vas 

 known by its capacity for attracting particles, could move 

 along a thread, ^ind he even transferred such a charge along 

 a hemp thread a thousand feet long suspended by threads of 

 silk placed at intervals. Then, ^vhen the thread was sus- 

 pended by metal wires, the charge vanished, being conducted 

 away by the wires. In 1729 Gray discovered that an electri- 

 fied glass tube would induce a charge in another tube close 

 by but not touching it, and a number of experimenters con- 

 tinued to study the nature of isolated electric charges and the 

 properties of static electricity. Electric machines for pro- 

 ,ducing powerful charge^ by means of induction were de- 



