164 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



excited cake of pitch and resin. This charge is re- 

 ceived by induction merely ; and as it expends none 

 of the electricity connected with the pitch, the suc- 

 cessive charges are precisely equal, a circumstance 

 which enabled Volta very simply to give a nume- 

 rical value to the amounts of electricity used in his 

 experiments, as they were given by one, two, or 

 more contacts with the electrophorus. The Con- 

 denser, an ingenious and useful instrument for ac- 

 cumulating small charges of electricity until they 

 attain a measurable amount, or cause divergence in 

 the electrometer, was described by Volta in 1782 in 

 the Transactions of the Royal Society, its construction 

 having occurred to him in following out the idea of 

 the electrophorus. This instrument was ultimately 

 of essential service in establishing his theory of Elec- 

 tro-motion. The theory of both the electrophorus 

 and condenser had been indicated by .ZEpinus some 

 time before Volta constructed them ; but he did not 

 apply them in the practical way which Volta did to 

 the improvement of his science. Volta being, be- 

 sides, unquestionably ignorant of JEpinus's labours, 

 has been generally and justly regarded as the real 

 inventor. Though both of these instruments depend 

 on the principle of induced electricity, Volta never 

 appears to have possessed correct views on that sub- 

 ject, but throughout his writings speaks of electrical 

 atmospheres, and uses other phrases of the old school 

 of electricians, showing a certain vagueness in his 

 conception which a study of the writings of his able 

 contemporaries, ^Epinus and Coulomb, would have 

 dissipated. 



(742.) But Volta's tact was unconnected with any tinge 

 St ^ of mathematical reasoning: his experimental ability, 

 Electrome- his caution, and his persevering devotion to one sub- 

 ter. ject, enabled him, however, to advance science in a 



different way. Even the simple Straw Electrometer 

 which he generally used, was tested by him with 

 such skill and care as to lead to correct results in 

 the measurement of small quantities of electricity. 

 M.Biot has indeed criticised his preference for so rude 

 an instrument, which depends, mathematically con- 

 sidered, upon repulsions of a very complicated cha- 

 racter ; but as Volta carefully tested its comparabi- 

 lity up to 30 of divergence, and found it proportional 

 to the force, there is no doubt that he was justi- 

 fied in relying on its use; and Arago, in opposition 

 to his colleague, maintains that Volta' s essay on the 

 Straw Electrometer is one of the best examples of 

 experimental research which can be put into the stu- 

 dent's hands. This instrument is described in the 

 first of a series of letters to Lichtenberg collected in 

 the first volume of his works ; the subject of these 

 letters being the Electricity of the Atmosphere. 

 (743.) The important experiments of Dalibard and Frank- 

 Atmosphe- ]j n> repeated with fatal consequences by Eichmann, 

 cit ' had demonstrated the perfect resemblance or rather 



identity of lightning and electricity. Lemonnier dis- 

 covered the fact of electricity being manifested when 

 no thunderstorm threatened, and even when the sky 

 was cloudless, and that it was subject to diurnal va- 

 riations of intensity. Beccaria farther ascertained 

 that with a clear atmosphere the electricity of the 

 air is always positive. De Saussure, Deluc, and 

 Volta continued the interesting enquiry. The 

 first, availing himself of the known action of points 

 to draw off electricity, connected his electrometer 

 with a pointed rod two or three feet in length. 

 Volta substituted for this the flame of a lamp pro- 

 ducing a heated current of air, which has a won- 

 derful power of drawing off electricity ; and he sug- 

 gested the employment of large fires during thunder- 

 storms in preference to metallic conductors. Volta 

 hesitates not to ascribe to the worshippers of Jupiter 

 Tonans the secret intention to draw off the electricity 

 of heaven by the action of the flames on the altar. 1 

 Arago has ingeniously suggested that a statistical 

 enquiry as to the frequency of thunderstorms in the 

 neighbourhood of extensive iron-smelting furnaces 

 might test the value of this safeguard. The straw 

 electrometer which Volta connected'with his appara- 

 tus was capable (by a gradation of instruments of 

 greater or less delicacy) of measuring numerically 

 the force of charges from 1000 to 2000 units. 



That the chief source of atmospheric electricity is (744.) 



evaporation appears first to have occurred to Volta, 



, r i r- i-i ' poration. 



and to have been first demonstrated by experiments 



made either with him or by his suggestion, at Paris 

 in 1780, by Lavoisier and Laplace. The history is 

 given by Volta himself in a paper in the Philosophical 

 Transactions for 1782. When water is thrown upon 

 an insulated heated body so that evaporation takes 

 place, or when hot coals are thrown into an insulated 

 vessel of water, the hot body is usually found to be 

 electrified negatively. Volta has very candidly stated 

 the inversions of effect which occasionally occur, and 

 which still throw some doubt upon the precise signi- 

 ficance of this very important experiment. Later 

 experimenters have thought that absolutely pure 

 water developes no electricity: this, however, will 

 not affect the validity of the explanation of the origin 

 of atmospheric electricity. To prevent misappre- 

 hension it may be observed that the astonishing de- 

 velopement of electricity from high-pressure steam 

 escaping through a small aperture, as lately observed 

 by Mr Armstrong, appears, from the experiments of 

 Dr Faraday, to depend on an entirely different cause. 

 Of Volta's electrical theory of Hail we cannot now 

 stop to speak. 



Volta, who understood chemistry and who always (745.) 

 took a peculiar interest in the inflammable gases, ^"jj" 

 contrived the Eudiometer which is often erroneously me t er 

 called Cavendish's, having been frequently used by 

 that philosopher. The amount of oxygen in the air 



1 Opere, vol. i., part 2., p. 205. Volta's contrivance dates from the beginning of 1787. Bennett imagined it independently. 



