108 ,Prof. C. Bohn on Negative Fluor eseence 



questioned whether external influences may not cause such bodies 

 frequently to pass from one form to the other, and consequently 

 through the typical parabolic orbit. And so the liquid condi- 

 tion of matter may perhaps be most scientifically described as 

 " unstable." Many substances do pass immediately from solid 

 to gas on an increase of temperature ; and where this is not the 

 case, liquids whose surfaces are in contact with gas undergo a 

 constant evaporation, which is accelerated either by an increase 

 of temperature or a decrease of external pressure— by anything, 

 in short, which disturbs the equilibrium in the hyperbolic direc- 

 tion. In a similar manner, if the equilibrium be disturbed in the 

 elliptic direction, the liquid will at once become solid. This is 

 exemplified by the experiment of water reduced while in a state 

 of perfect rest to a temperature many degrees below freezing- 

 point : it still remains liquid till slightly agitated, when ice is 

 suddenly formed. 



During the metamorphosis of matter from solid to liquid and 

 from liquid to gas a large amount of heat is expended. In 

 ancient language caloric became latent ; in modern phraseology 

 it is held that this heat is consumed in conferring potential 

 energy upon the atoms of the substance. If sensible heat is 

 motion, latent heat is position. More definitely, according to 

 our theory, the non-thermometric heat is expended in changing 

 the form of the orbit from ellipse to parabola, from parabola to 

 hyperbola. 



XIV. On Negative Fluorescence and Phosphorescence. 

 By Professor C. Bohn * . 



1. TT is well known that the phenomenon of fluorescence has 

 J- been observed in the case of a great number of substances, 

 and that it has been described by many physicists. But hitherto 

 the law established by Professor Stokes f, according to which 

 the refrangibility of the fluorescent rays never exceeds that of 

 the rays which excite the fluorescence, has not been gainsaid. 

 In fluorescence, as hitherto known, the process is generally re- 

 garded as consisting in the transformation of rays of greater re- 

 frangibility into rays of less refrangibility. In accordance with 

 this conception, M. E. Becquerelf hoped to be able to detect by 

 experiment the transformation of red rays of light into less re- 

 frangible rays of heat, but he did not succeed in doing so. In 



* Translated from PoggendorfF 's Annalen, vol.cxxx. p. 367 (April 1867). 

 t " On the Change of Refrangibility of Light," § 80, Phil. Trans. 1852, 

 and elsewhere. 



X Annates de Chimie et de Physique, ser. 3. vol. lv. p. 5. 



