DISCOVERY OF CONTACT ELECTRIFICATION 447 



in exactly the same manner, produce a greater or less quantity of electricity. 

 Thus also, by only heating or cooling the metals, the electricity may be varied 

 in quantity and even in quality. 



I am inclined to suspect, that different bodies have different capacities for 

 holding the electric fluid, as they have for holding the elementary heat; if how- 

 ever the experiments relative to this subject be carefully tried, under all the 

 variety of circumstances which the combination of the above-mentioned causes is 

 capable of producing, I do not doubt but that all the phenomena observed in the 

 preceding pages may hereafter be reconciled to one, or to a few, simple laws, 

 which will at the same time assist the farther investigation of the science of 

 electricity. 



These experiments of Cavallo's could not have been made later tlian 

 the year of publication, viz., 1795. While Cavallo says in his discus- 

 sion of animal electricity that Volta suspects that the phenomena of 

 muscle contraction which Galvani and he were studying might be 

 caused by the contact of the two dissimilar metals which were used in 

 making the connection between the muscle and the nerve in many of 

 their experiments, yet Volta seems not to have actually experimented 

 with contact electrification until after the publication of Cavallo's 

 treatise, and then to have begun with the repetition of Bennett's experi- 

 ments made ten years before. 



There seem to be many diverse statements as to how Volta arrived 

 at his theory of contact electricity, but his own story of it is given in a 

 so-called letter to Dr. Gren, which was published in volumes III. and 

 IV. of Gren's Neues Journal der Physih, in the years 1796-98. These 

 journals are not accessible to the present writer, so their exact date can 

 not be given, though Vol. IV. was concluded in 1798. Volta's letters 

 to Gren are translated in the Philosophical Magazine of 1799, from 

 which the extracts here given are quoted. 



In the first part of Volta's letter, which was published in A^ol. III. 

 of Gren's Journal, Volta says: 



The contact of different conductors, particularly the metallic, including 

 pyrites and other minerals as well as charcoal, which I call dry conductors, or 

 of the first class, with moist conductors, or conductors of the second class, 

 agitates or disturbs the electric fluid, or gives it a certain impulse. Do not ask 

 in what manner: it is enough that it is a principle, and a great principle. This 

 impulse, whether produced by attraction or any other force, is different or unlike, 

 both in regard to the different metals and to the different moist conductors, so 

 that the direction, or at least the power, with which the electric fluid is impelled 

 or excited, is different when the conductor A is applied to the conductor B, and 

 to another C. In a perfect circle of conductors, where either one of the second 

 class is placed between two different from each other of the first class, or, con- 

 trariwise, one of the first class is placed bet^j:een two of the second class different 

 from each other, an electric stream is occasioned by the predominating force 

 either to the right or to the left — a circulation of this fluid, which ceases only 

 when the circle is broken, and which is renewed when the circle is again ren- 

 dered complete. 



