3H HISTORY OF SCIENCE. 



of the air-pump (page 172), to whom also science is indebted for the 

 first electrical machine. Guericke simply made a globe of one of the 

 substances recognized by Gilbert as possessing the same property as 

 amber, and this globe the Magdeburg physicist caused to turn on an 

 axle, so that it could readily be rubbed. In a book published in 1672 

 Guericke describes the mode of preparing the machine thus: "Take 

 a spherical glass bottle as big as a child's head, fill it with small pieces 

 of sulphur, and bring it near the fire so as to melt the sulphur. Then 

 Jet it become cold, and break the glass in order to get out the sphere 

 of sulphur, which you will keep in a dry place. If you like you may 

 make a hole through it, so that it can be turned round on an iron 

 axis, and the globe will then be ready for use." Guericke first noticed 

 the electric spark, which this arrangement, so much larger than the 

 small pieces of material used by Gilbert, permitted him to observe. 

 He also remarked that light substances, which had been attracted by 

 the excited sulphur, were afterwards repelled until they had touched 

 some other body. When a novel observation of this kind is made by 

 a person of a scientific turn of mind, he almost always seeks to find in 

 the phenomena some analogy with other phenomena with which he 

 is familiar. Sometimes the similarities between apparently different 

 cases are so disguised as to require a very rare order of intellect to 

 perceive them ; in other cases they may be sufficiently obvious. The 

 discoverer is, however, liable to be placed on the wrong track by mis- 

 leading analogies. And this is what happened in the present case 

 to Guericke, who saw in these attractions a perfect imitation, as he 

 thought, of the attraction which the earth exercises on bodies near it. 



Guericke's machine, if such it can be called, possessed very feeble 

 power. The spark could only been seen in the dark, and the sound 

 accompanying it was audible only to an ear placed very near the sul- 

 phur globe. 



Much more powerful effects were produced by a machine arranged 

 by HAWKSBEE (16 . . 17 . .), who also made considerable additions 

 to our knowledge of electrical phenomena, though he did not at first 

 suspect that electricity was the agent concerned in the phenomena he 

 observed. In the year 1705 Hawksbee found that when mercury is 

 shaken in glass vessels, light is produced. This fact, indeed, was 

 known before; but Hawksbee tried the effect of shaking the mercury 

 in a bottle from which the air was exhausted, in order to discover if 

 the air was in any way the cause, of the luminous appearance. He 

 was somewhat surprised to find that when the bottle was partly ex- 

 hausted the light was more vivid than before; but though he con- 

 cluded that friction was the cause of the phenomenon, he was unable 

 to determine whether or not the presence of some air was necessary. 

 By many experiments he discovered that the rubbing together of 

 various substances was capable of producing light, as, for instance, 

 glass or amber rubbed with flannel. Continuing his experiments, he 



