578 SCIENTIFIC RECORD FOR 1885. 



bles together ? Under what circumstances can we conceive a group of 

 waves of light to begin gradually and to end gradually ? You know 

 what takes place in the excitation of a fiddle string or of a tuning fork 

 by a bow. The vibrations gradually get up from zero to a maximum, 

 and then, when you take the bow off, gradually subside. I cannot see 

 anything like that in the source of light. On the contrary, it seems to 

 me to be all shocks — a sudden beginning and gradual subsidence." But 

 it is the double refraction difficulty which is most serious. When the 

 medium is displaced during wave propagation In a doubly refracting 

 crystal, the return force must depend on the direction of vibration, and 

 not on the plane of distortion, as theory indicates. Rankine's suggestion 

 of a cavity in the luminiferous ether having a massless, rigid lining, and 

 containing a massive, heavy molecule with fluid round it, seemed to solve 

 the difficulty. But the form of wave surface deduced from such a hy- 

 pothesis does not agree with Fresnel's, asEayleigh has shown. "It ap- 

 pears, then," says Professor Forbes, in his excellent r^sum^ of the lectures, 

 " that after all the labor which has been expended upon the wave theory 

 of light it fails absolutely, and as it seems hopelessly, in two points of 

 primary importance. One is the extinction of the ray polarized by re- 

 flection ; the other is double refraction. In other matters we have 

 difficulties, but we can see a possible means of escape. Here there 

 seems to be none." {Nature, March, April, 1885, xxxi, 4G1, 508, 601.) 



At the Aberdeen meeting of the British Association, Stoney showed 

 that the mass of a molecule of hydrogen must be a quantity of the same 

 order as a decigram divided by 10^*, i. e., a twenty-fourth decigrammet, 

 which is the same as a twenty-fifth grammet. Hence the mass of the 

 chemical atom of hydrogen may betaken to be half of the twenty fifth 

 of the grammet. This value is based on the conclusion reached by 

 several physicists that the number of molecules in a cubic millimeter of 

 a gas at ordinary temperature and pressure is somewhere about a unit 

 eighteen (10'^); from which it can be shown that the number of mole- 

 cules per liter must be about a unit twenty -four (lO^*"). From this, to- 

 gether with the weight of a liter of hydrogen, the above value for the 

 mass of a molecule of hydrogen has been deduced. {Nature, November, 

 1885, XXXIII, 21.) 



In a paper on the identity of energy. Lodge has drawn some interest- 

 ing conclusions from an important memoir by Poynting, presented to 

 the Eoyal Society. In this memoir the author introduces " the idea of 

 continuity in the existence of energy ; so that whenever energy is trans- 

 ferred from one place to another at a distance it is not to be regarded 

 as destroyed at one place and recreated at another, but is to be re- 

 garded as transferred, just as so much matter would have to be trans- 

 ferred ; and accordingly we may seek for it in the intervening space 

 and may study the paths by which it travels." Lodge regards this new 

 form of the doctrine of conservation of energy as much simpler and 

 more satisfactory than the old one ; and he proves it rigidly and in- 



