320 ATMOSPHERIC ELECTEICITY. 



currents in bodies wliicli are poor conductors, by merely taking tte 

 precaution of slightly moistening tbera. In this way he obtained 

 very sensible deviations of the needle of a rheometer by fixing to the 

 extremities of the wire of the latter two sticks of moistened clay, one 

 of which, strongly heated, was placed in contact with the other, re- 

 maining at the ordinary temperature. " If we consider for an instant," 

 says M. Becquerel,* " a portion of the atmosj^here in a perfect calm, 

 and having througliout the same temperature, the state of equilibrium 

 of its electricity would not be disturbed ; but if, by any cause what- 

 ever, it should happen to be penetrated by a current of cold air, a 

 part will become cool, take the negative electricity, and the other 

 part the positive electricity. The contact of the molecules being 

 short in duration, by reason of the rapidity of the current, every one 

 of them would preserve a part of the electricity which is disengaged 

 during the change of the temperature. If the portions which have 

 become cooled enclose aqueous vapor, this will become condensed, im- 

 bibe electricity, and form a cloud charged with negative electricity. 

 When the cold current of air contains vapor, a cloud will be found pos- 

 sessing positive electricity. It has been observed that in general the air 

 which is at a certain distance from houses and trees, possesses positive 

 electricity. This is evident, i'or the cold air which is found in contact 

 with the earth, after being heated at its expense, is raised on account 

 of a less specific gravity, and bears with it the positive electricity 

 which it has taken during its heating." It is probable that effects of 

 this kind take place in the atmosphere. Experience and observation 

 only can decide in what degree they concur in the production of the 

 electrical phenomena of the air. 



We see from the foregoing how little we have advanced in our knowl- 

 edge of the origin of the positive electricity of the air in clear weather. 

 In examining this question, M. Peltierf offers a new opinion, accord- 

 ing to which the air is completely deprived of electricity, while 

 all the electric phenomena of the atmosphere derive their source from 

 the negative electricity of the earth. In describing the experiments 

 previously mentioned, in which an electrometer diverged positively 

 when it was raised, and negatively when lowered, he presents his 

 mode of considering atmospheric electricity, '■ These experiments," 

 says he, "demonstrate that the earth acts as a body powerfully nega- 

 tive, and the celestial space as a body strongly positive, and all the 

 bodies interposed between them become electrified by induction, accord- 

 ing to their position and relations with the earth, and not by contact 

 with the air. When a jjerson is in an uncovered place, and under a 

 clear sky, his head is in a negative state, as are the upper ends of 

 all bodies, such as the tops of trees. This negative tension changes 

 with our position, with the state of the sky, and the shelter which 

 cover us. This electric induction is proved by placing the hand 

 raised above an insulated globe screwed to the upper part of the elec- 

 trometer. The leaves of gold diverge somewhat negatively. This 

 effect is reproduced with force when there prevails one of those red- 



* Annales dc Chimie el de Physyique, torn. 5LI, page 372. 

 Compte's Kendus, torn. XII, page 3(i7 1841. 



