xlvi 



THE PROGRESS OF 



of the earth. That part of the earth nearest it 

 will become negative. The negative electricity 

 will accumulate, and the positive will be repelled 

 to a distance. Let the electricity be suddenly 

 discharged from the cloud, the positive electricity 

 will return bark to the part of the earth below 

 the cloud with impetuosity, while the negative 

 electricity will recede with equal velocity. 

 Hence it may happen that animals may be de- 

 stroyed by such a returning stroke, even when 

 no thunder is near them. A remarkable instance 

 of this kind took place at Coldstream in the year 

 1786, when a man and two horses were killed by 

 the returning stroke, soon after passing the 

 Tweed. 



Coulomb appeared as an electrician about the 

 year 1785, and added more important facts than 

 any who had appeared after the time of Franklin. 

 He began by constructing a very delicate electri- 

 cal balance, which measured the attractive and 

 repulsive forces by the tort ion of a very fine 

 wire. The three discoveries which he made, by 

 means of this balance, constitute, in fact, the 

 basis of all electrical theory. These discoveries 

 are the following : 



1. The attractions and repulsions of electrical 

 bodies vary inversely as the square of their dis- 

 tances. 



2. When insulated bodies are charged with 

 electricity, they gradually lose that charge. This 

 is partly owing to the surrounding atmosphere 

 which never being absolutely free from conduct- 

 ing particles, these particles gradually get posses- 

 sion of the electricity, and carry it off. It is 

 partly owing also to the electric bodies, which 

 serve as insulators. There is, probably, no sub- 

 stance absolutely impervious to electricity ; 

 though some are more and some less so. Hence 

 the electricity gradually passes off along the in- 

 sulated body. Coulomb determined the effect 

 of both these causes, and thus put it in the power 

 of electricians to calculate how much electricity 

 an insulated body loses in a given time. 



3. When electricity is accumulated in any 

 body, the whole of it is deposited on the surface, 

 and none of it penetrates into the interior. So 

 that a hollow sphere, how thin soever its walls 

 may be, is capable of receiving as great an elec- 

 tric charge as a solid body of the same dimen- 

 sions. 



From the various experiments of Davy in 

 1822, there is reason to conclude that the electric 

 spark is merely evolved by the sudden condensa- 

 tion of air, or any elastic fluid through which the 

 electricity passes. For though electricity passes 

 readily through an absolute vacuum, no spark 

 whatever can be perceived. 



Electricity may be evolved in some bodies by 

 beat as well as friction. This is the case with the 



tourmalin, the electrical properties of which were 

 investigated by Bergman and .-Kpinus. When a 

 transparent tourmalin is heated to 212, one of its 

 ends becomes positive, and the other negative, 

 while the centre of the prism is a neutral point, 

 exhibiting no signs of electricity whatever. 

 From 212* the electric energy increases rapidly, 

 till the temperature sinks to 156*, when it is a 

 maximum. From 156 to 104 it remains nearly 

 stationary, and sinks rapidly from 104 to 70, 

 when it is nearly the same as at 212. If we 

 break the tourmalin in two while in a state ol 

 excitement, each of the fragments will be found 

 to possess precisely the electrical properties ot 

 the whole crystal. 



The tourmalin is not the only mineral which 

 exhibits these properties. Four or five other 

 crystallized minerals possess similar properties. 



Becquerel has lately shown, that when two 

 elastic bodies are pressed strongly against eadi 

 other, provided both be insulated, they acquire 

 different states of electricity, the one becoming 

 positive and the other negative. Iceland spar, 

 sulphate of lime, fluor spar, sulphate of barytes, 

 when pressed against cork become positively 

 electrided, while the cork becomes negative. 

 But cork becomes positive when pressed against 

 cyanite and retinasphaltum. Electricity thus 

 evolved continues a long time. 



Pouillet has lately ascertained, that when char- 

 coal is burned, the portion of the piece which is 

 unconsumed becomes charged with negative elec- 

 tricity, while the carbonic acid gas formed is 

 positive. When hydrogen is burned, the air round 

 the flame at some distance is positive, but the 

 centre of the flame is negative. 



About the beginning of the present century, a 

 new and most important electric instrument was 

 contrived by Volta, which has been employed 

 with the most fortunate success in promoting the 

 science of chemistry. When two different metals, 

 zinc, for example, and copper, are placed in con- 

 tact, and then separated, the zinc is electrified 

 positively and the copper negatively. If we take 

 a number of pairs of these two metals, placing 

 each pair in contact and in the same order, and 

 introduce between each pair a card moistened 

 with wet salt-water, a current of electricity is 

 evolved. If we apply the wet finger of one 

 hand to the top of such a pile, and the wet finger 

 of the other hand to the bottom of the pile, we 

 immediately feel an electric shock, the intensity 

 of which increases as the number of pairs of 

 plates. With 100 pairs the shock is severe. 

 Such was the origin of what is called the Gal- 

 vanic or Voltaic battery. The plates are usually 

 about three or four inches square. Each zinc 

 plate is surrounded with copper, distant from it 

 about a quarter of an inch, except at the top, 



