LESSONS IN ELECTRICITY. 



nomenon to its true cause, namely, the 

 friction between mercury and glass in the 

 highly rarefied air. John Bernoulli! ridi- 

 culed Hauksbee's explanation. But 

 truth outlives ridicule, and it is now uni- 

 versally admitted that Hauksbee was 

 right. 



Hauksbee also made the following ex- 

 periment, which, as shown by Riess, is 

 explained by reference to the principle of 

 induction. A hollow glass globe was 

 mounted so as to be capable of quick 

 rotation. It was exhausted, and while 

 it rotated the hand was placed against it 

 in the dark. It was positively electri- 

 fied by the hand. This positive electricity 

 acted inductively on the gla?s itself, at- 

 tracting its negative, but discharging its 

 positive as a luminous g'ow through the 

 rarefied air within. Haukshee was able 

 to read by the light thus produced. 



By such experiments it was shown that 

 rarefied air favored the passage of elec- 

 tricity. Dry air is in fact an insulator, 

 which must be broken through to pro- 

 duce the electric spark. Through an ex- 

 hausted glass tube six feet long a dis- 

 charge freely passes which would be in- 

 competent to leap over the fiftieth part 

 of this interval in air. But whereas the 

 spark in air is dense and brilliant, the 

 discharge in vacuo fills the exhausted 

 tube with a diffuse light. 



(It is here worthy of remark that at 

 a very early period Grummert, a Pole, 

 proposed the employment 'of this diffuse 

 electric light to illuminate coal mines a 

 notion which has been revived in our 

 day. The light in this form is not com- 

 petent to ignite the explosive gases which 

 produce such terrible disasters in mines.) 



Priestley, in his " History of Electric- 

 ity," thus describes the light in vacuo. 

 " Take a tall receiver, very dry, and in the 

 top of it insert with cement a wire not 

 very acutely pointed, then exhaust the 

 receiver and present the knob of the wire 

 to the conductor, and every spark will 

 pass through the vacuum in a broad 

 stream of light, visible through the whole 

 length of the receiver, be it ever so tall. 

 This stream often divides itself into a 

 variety of beautiful rivulets, which are 

 continually changing their course, uniting 

 and dividing again in the most pi-easing 

 manner. If a jar be discharged through 



this vacuum, it gives the appearance of a 

 very dense body of fire, darting directly 

 through the centre of the vacuum with- 

 out ever touching the sides." 



Cavendish employed a double barome- 

 ter-tube, bent into a form of a horseshoe, 

 with its curved portion empty, to show 

 the passage of electricity through a 

 vacuum. It is really not the vacuum 

 which conducts the electricity, but the 

 highly attenuated air and vapor which 

 fill the space above the barometric 

 columns. When the mercury employed 

 is carefully purged of air and moisture 

 by previous boiling, the space above 

 the mercury, as proved by Walsh, 

 De Luc, Morgan, and Davy, is wholly 

 incapable of conducting electricity. 

 Similar experiments have been made in 

 the laboratory of Mr. Gassiot, to whom 

 we are indebted for so many beautiful 

 electrical experiments. Professor Dewar 

 has also brought his experimental skill to 

 bear with success upon this subject. 



Electricity, therefore, dees not pass 

 through a true vacuum ; it requires pon- 

 derable matter to carry it. If a gold- 

 leaf electroscope be kept at a distract 

 from al] conductors, it may be kept 

 charged for an almost indefinite period 

 in a good air-pump vacuum. 



The matter rendered thus luminous by 

 the electrical discharge is attracted and 

 repelled like other electrified matter. " A 

 finger," says Priestley, " put on the out- 

 side of the glass will draw it [the lumi- 

 nous stream] wherever a person pleases. 

 If the vessel be grasped with both hands, 

 every spark is felt like the pulsation of a 

 great artery, and all the fire makes to- 

 wards the hands. This pulsation is felt 

 at some dirtance from the receiver ; and 

 in the dark a light is seen betwixt the 

 hands and glass." 



" All this," continues the historian of 

 electricity, " while the pointed wire is 

 supposed to be electrified positively ; if 

 it be electrified negatively the appearance 

 is remarkably different. Instead of 

 streams of fire, nothing is seen but one 

 uniform luminous appearance, like a 

 white cloud, or the milky-way on a clear 

 starlight night. It seldom readies the 

 whole length of the vessel, but is gen- 

 erally only like a lucid ball at the end of 

 the wire." 



Of the two appearances here described 



