88 COSMOS. 



duction, to use a conventional mode of expression, passed 

 through the moist earth, we should seem to be justified in 

 concluding that the velocity of the transmission of electricity 

 depends upon the nature as well as the dimensions* of the 

 medium. Bad conductors in the voltaic circuit become more 

 powerfully heated than good conductors ; and the experi- 

 ments lately made by Eiessf show that electric discharges 

 are phenomena of a very various and complicated nature. 

 The views prevailing at the present day regarding what is 

 usually termed " connection through the earth" are opposed 

 to the hypothesis of linear, molecular conduction between 

 the extremities of the wires, and to the conjectures of the 

 impediments to conduction, of accumulation, and disruption 

 in a current, since what was formerly regarded as interme- 

 diate conduction in the earth is now conjectured to belong 

 exclusively to an equalization or restoration of the electric 

 tension. 



Although it appears probable, from the extent of accura- 

 cy at present attainable in this kind of observation, that the 

 constant of aberration, and, consequently, the velocity of 

 light, is the same for all fixed stars, the question has fre- 

 quently been mooted whether it be not possible that there 

 are luminous cosmical bodies whose light does not reach us, 

 in consequence of the particles of air being turned back by 

 the force of gravitation exercised by the enormous masses 

 of these bodies. The theory of emission gives a scientific 

 form to these imaginative speculations.^ I here only refer 



* See PoggendorflPs Annalen, bd. Ixxiii., 1848, s. 337, and Pouillet, 

 Comptes Rendus, t. xxx., p. 501. 



t Riess, in PoggendorfTs Ann., bd. 78, s. 433. On the non-conduc 

 tion of the intermediate earth, see the important experiments of Guille- 

 miu, Sur le courant dans une pile isolte ct sans communication entre let 

 pdles in the Comptes Rendus, t. xxix., p. 521. " Quand on remplace 

 un fil par la terre, dans les telegraphes electriques, la terre sort plut6t 

 de reservoir commun, quo de moyen d'union entre les deux extremi- 

 tes du fil." " When the earth is substituted for half the circuit in the 

 electric telegraph, it serves rather as a common reservoir than as a 

 means of connection between the two extremities of the wire." 



t Madler, Astr., a. 380; also Laplace, according to Moigno, Repertoire 

 d'Optique Moderne, 1847, t. i., p. 72 : " Selon la theorie de l'6mission 

 on croit pouvoir demontrer que si le diametre d'une 6toile fixe serait 250 

 fois plus grand que celui du soleil, sa densite restant la meme, 1'attrac- 

 tion exercee a sa surface detruirait la quantite de mouvement, de la 

 moldcule lumineuse Praise, de sorte qu'elle serait invisible a de gramles 

 distances." " It seems demonstrable by the theory of emission that if 

 the diameter of a fixed star be 250 times greater than that of the sun 

 its density remaining the same the attraction exercised on the surface 



