88 COSMOS. 



duction, to use a conventional mole 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- 

 mec.ts lately made by Riessf show that electric discharges 

 are phenomena of a very various and complicated nature 

 Ihe \'i3ws prevailing at the present day regarding what is 

 usually termed " connection through the earth" are opposed 

 to the hypothesis of linear, molecular conduction betvi'^een 

 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 

 consta/tt of aberration, and, consequently, the velocity of 

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

 quently been mooted Avhether 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 PoggendorfT's Annalen, bd. Ixxiii., 1818, s. 337, and Pouiilet, 

 Comptes Rendns, t. xxx., p. 501. 



t Riess, in roggendorft''s Ann., bd. 78, s. 433. On the non-conduc 

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

 min, Sur le courant dans une pile isoMe ct sans communication enire let 

 poles in the Comptes Rendns, t. xxix., p. 521. " Quand on remplace 

 un fil par la terre, dans les telegraphes electriques, la terre sert pkuot 

 de reservoir commun, que de moyen d'union eiitre 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 resen'oir than as a 

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



X Madler, Astr., s. 380; aUD Laplace, according to Moigno, RSperioirt 

 d'Optique Moderne, 1847, L i., p. 72 : " Selon la theorie de I'emission 

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

 fois plus grand que celui du soleil, sa densite restant la me me, I'attrac- 

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

 molecule lumineuse emise, de sorte qu'elie serait invisible a de grandes 

 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— 

 it3 density remaining the same — th(' attraction exercise.! on th*.' sTn'facf 



