220 Dr. J. Moser on the Microphonic 



the efficiency of the selenium cell, has been the second cause 

 which masked the microphonic action of the cell. These are 

 changes in the material ; they concern rather the chemistry of 

 selenium. They are indeed purely chemical if we view the exist- 

 ence of selenium in four allotropic modifications as a chemical 

 quality. Into these chemical changes I shall enter here only so 

 far as it is necessary for the proof that they are not essential 

 to the microphonic action of the selenium cells, since my aim 

 now is the physical one of correlating two forces. 



When, now, we no longer consider the selenium cells espe- 

 cially, but selenium in general, we find that light can produce 

 in it, if the selenium is an element and pure *, no other che- 

 mical changes than those which induce the transformation of 

 one of the allotropic modifications into another one. But 

 such a transformation is connected with development of 

 energy; for in 1851 M. Hittorfj observed a rise of tempe- 

 rature of 90 degrees when the vitreous modification changed 

 into the crystalline; and Regnault J (1856) in a similar case, 

 observed an elevation of temperature of 130 degrees. With 

 a proper arrangement (that is, in a closed circuit) we shall get, 

 instead of the development of heat, an electric current — just 



. as, for instance, two solutions of the same salt, but of dif- 

 ferent concentration, on being mixed together, give a deve- 

 lopment of heat, but, when brought into a circuit in a proper 



; manner, produce electricity equivalent to this heat. 



I have already mentioned above that Prof. W. Gr. Adams thus 

 observed electromotive forces on illuminating selenium con- 

 nected only with a galvanometer, but that generally the pho- 

 tophonic cells do not give such a development of electricity, 

 and that therefore this property of selenium cannot be used 

 for the explanation of the efficiency of the photophonic cells. 

 (Here I will add that of course thermoelectric currents can be 

 produced by differential heating of the two junctions. In 

 order to get such a great difference of heating, the cell must 

 be exposed to greater heat. Thus I brought near the focus of 

 the sun's rays only one small part of the cell, and found a 

 small deflection of 10 divisions, or about half a centimetre.) 



* On melting selenium, a grey film was formed on the surface. This 

 being removed by a platinum spatula, the amorphous selenium showed a 

 brilliant surface ; and only such selenium was employed, in order to get 

 congruent results. 



Selenium crystals of 3 millim. length were obtained by sublimation 

 on the cover of the crucible. At first amorphous selenium condensed ; 

 afterwards these sublimated crystals, which Were insoluble in water, Were 

 formed. 



t Pogg. Ann. lxxxiv. p. 216. 



% Ann. Chim. et Phys. (3) xlvi. p. 284. 



