58 



ELECTRICITY. 



mon electrical machine is derived prin- 

 cipally from this source. When he 

 employed as the rubbing substance an 

 amalgam of silver or of platina, which 

 are metals very little subject to oxida- 

 tion, he could obtain no electricity. An 

 amalgam of tin, on the other hand, sup- 

 plied" a large quantity of electricity. 

 Zinc acts still better than tin ; but the 

 best amalgam for this purpose is made 

 with both tin and zinc, a mixture which 

 oxidates more readily than either metal 

 separately. As a further trial whether 

 oxidation assists in the production of 

 electricity, a small cylinder with its 

 cushion and conductor was arranged in 

 a vessel so contrived that the contained 

 air could be changed at pleasure. After 

 ascertaining the degree of excitement 

 produced in atmospheric air, carbonic 

 acid was substituted, but the excitement 

 could not be renewed ; while it was im- 

 mediately reproduced on the readmisston 

 of common air. It must be acknow- 

 ledged, however, that Sir H. Davy, in 

 repeating these experiments, arrived at 

 opposite results ; for he states, that the 

 machine acted equally well in hydrogen 

 gas as in atmospheric air, and was even 

 more active in carbonic acid gas, a cir- 

 cumstance which he attributes to the 

 greater density of this gas. 



(215.) Electricity is often developed 

 by processes quite independent of che- 

 mical changes. This is evident from its 

 production by the friction of two bodies 

 of the same kind upon one another, 

 as has been already noticed, ( 35 ;) and 

 also by the strong electricity which is 

 manifested on the separation of the 

 parts of the same body. r lhus, if a 

 piece of dry and warm wood be suddenly 

 -rent asunder, the two surfaces which 

 have separated are found to be elec- 

 trified, the one positively, the other 

 negatively ; and a flash of light is per- 

 ceived if the experiment be made in the 

 dark. The same phenomenon is ob- 

 served when the plates of mica (Mus- 

 covy glass) are suddenly torn asunder ; 

 and even when a stick of sealing-wax is 

 broken across ; the two surfaces of 

 fracture being in each case positive and 

 negative respectively. Dr. Brewster 

 discovered that the fracture of the un- 

 annealed glass tears, called Prince Ru- 

 pert's drops, was attended with the evo- 

 lution of electrical light, which pervaded 

 the whole drop, so that its form was 

 distinctly visible in the dark. The light 

 appears even when the expeiiment is 

 made under water. 



(216.) There is every reason to pre- 



sume that electricity is essentially con- 

 cerned in the processes that are carried 

 on in the living system both of animals 

 and vegetables. In the animal economy 

 more particularly, the operation of this 

 agent is indicated in the processes of 

 secretion, in the actions of the muscles 

 and nerves, and probably, indeed, in all 

 the vital functions. There are several 

 kinds of fish, which are endowed with 

 the power of accumulating large quan- 

 tities of electricity, which they can dis- 

 charge at pleasure through conducting 

 bodies that come in contact with them, 

 and thus communicate powerful shocks. 

 This power is possessed in an eminent 

 degree by the torpedo, which is a species 

 of ray ; but it is also met with in the 

 Gymnotus electricus, the Silurus electri- 

 cus, the Trichiurus indicus, and the 

 Tetraodon electricus. But as this, as 

 well as other subjects relating to animal 

 electricity, involve considerations which 

 properly belong to Galvanism, we must 

 defer treating of them until this branch 

 of electrical science is before us. 



CHAPTER XV. 

 Electricity of the Atmosphere. 



(217.) As the subject of atmospheric 

 electricity is more especially a branch of 

 the science of Meteorology, we shall con- 

 tent ourselves, in this place, with a very 

 brief outline of the principal facts relat- 

 ing to it. 



(218.) The atmosphere is very gene- 

 rally in an electrical state. This may be 

 ascertained by employing a metallic rod 

 elevated to some height above the ground, 

 and communicating at its lower end, 

 which should be insulated, with an elec- 

 troscope. In order to collect the elec- 

 tricity of the higher regions cf the air, a 

 kite may be raised, in the string of which 

 a slender metallic wire should le inter- 

 woven, so as to conduct the electricity. 

 If the electroscope be sufficiently sensi- 

 ble it will usually indicate the prevalence 

 of positive electricity in the atmosphere, 

 the intensity of which increases accord- 

 ing as the stratum examined is more 

 elevated. In the ordinary slate of the 

 atmosphere its electricity is invariably 

 found to be positive : and is stronger in 

 winter than in summer; and dining the 

 day than the night. From the time of 

 sunrise it increases for two or three 

 hours, and then decreases towards the 

 middle of the day, being generally weak- 

 est between noon and four o'clock. As 

 the sun declines its intensity is again 

 augmented, till about the time of sun- 



