M. E. Pringsheirn on the Radiometer, 101 



oxides are well-known examples, the specific gravity of which 

 increases with the amount of heat to which they are subjected, 

 while their solubility in acids diminishes. 



I believe, then, that it will be possible to connect together 

 the molecular volume and the solubility of solids, just as the 

 molecular volume and the boiling-point of liquids have been 

 shown to be connected. The problem is necessarily a more 

 complex one, as we have to deal not only with the mutual 

 attraction of the molecules of the solid, but also with that of 

 the water for the solid, which varies not only with the tem- 

 perature, but also with the nature of the solid. However, 

 some experiments I have already made lead me to believe that 

 such determinations are possible. 



Since the above paper was written, my attention has been 

 drawn to an article on Solution by Dossios*, which I had 

 known only in abstract : our theories have much in common ; 

 but Dossios's is unsupported by experimental evidence. 



XVIL On the Radiometer. By Eenst Pringsheim f. 



THE uncertainty and differences of opinion which still exist 

 respecting the cause of the motion in the radiometer 

 and similar apparatus appear to arise from the circumstance 

 that in fact the motion of these instruments is essentially in- 

 fluenced by several different things, the actions of which are 

 reciprocally so complementary and disturbing that by the 

 complexity of the different results the conformity to law which 

 prevails among them is often obscured. 



If we ask what are the things which may have influence 

 upon radiometer-motion, they are, first, the different parts of 

 the apparatus itself — namely the glass case, the enclosed gas, 

 and the vanes. Therefore the only right way to study the 

 mode of working of radiometric apparatus will be, to separate 

 as much as possible the effects of these different parts experi- 

 mentally, and to treat each of them singly. 



Apparatus. 



For this purpose the ordinary radiometers are extremely 

 unsuitable, as in them the phenomena are very much com- 

 plicated by the moving force always appearing simultaneously 



* " Zur Theorie der Losungen," Vierteljahrsschrift der zurichischen Na- 

 turforschenden Gesellschaft, xiii. pp. 1-21 ; Jahresbericht, 1867, pp. 92-95. 

 t Translated from Wiedemann's Annalen, 1883, No. 1, pp. 1-32. 



