NATURAL PHILOSOPHY. 215 



does expand as it is cooled below 39 Fahr. After the mass is frozen, the 

 thermal motions of or among its particles can scarcely be motions involv- 

 ing any excursions of their polar axes ; and whatever they are, a diminution, 

 of them will again begin to produce the natural and ordinary effect of a 

 diminution of what Sir Humphrey Davy has well called " repulsive mo- 

 tion ; " that is, will allow the mass to contract. The Dynamical theory of 

 heat, then, while it obviously shows the true reason of the natural and 

 ordinary phenomenon of the contraction of a mass, whether fluid or solid, 

 may consistently explain the contraction of liquid water as it is cooled 

 down to 39, its gradual expansion as it is cooled farther, its sudden ex- 

 pansion in freezing, and the contraction which the solid ice experiences 

 when the lowering of temperature is continued. 



Prof. Stokes addressed the Section with reference to a passage in the 

 Report. He remarked that there was one phenomenon relating to light 

 which was strictly analogous to, if not identical with, (he himself believed 

 identical with,) the gradual emission of radiant heat by a body which 

 had been warmed by exposure to radiant heat : he alluded to phospho- 

 rescence. This phenomenon was very intimately allied to another, exhib- 

 ited by a solution of sulphate of quinine, by glass colored by oxide of 

 uranium, c., which has been termed fluorescence. In the latter phenom- 

 enon certainly the law appeared to be general, that the light emitted was 

 of lower refrangibility than the rays affecting the medium ; and the same 

 appeared to be at least usually true in the case of phosphorescence. Dr. 

 Draper had indeed stated that Canton's phosphorus was rendered luminous 

 by the rays from incandescent lime after traversing a strong solution of 

 bichromate of potash. Prof. Stckes stated that he had repeated this 

 experiment, but had not obtained the same result, the rays in his own 

 experiments having proved inefficient after traversing the solution. It 

 seemed worthy of investigation to examine whether the heat emitted by a 

 body which had been heated by rays of some particular refrangibility 

 consisted, in all cases, exclusively of rays of lower refrangibility. Much 

 progress, he conceived, would be made in our knowledge of the subject of 

 radiant heat, if the absorbing power of several common substances, such 

 as water, alum, &c., frequently employed in researches on heat, were 

 determined for each degree of refrangibility of radiant dark heat in partic- 

 ular. This determination would require, first, the formation of a pure 

 heat spectrum ; second, the rendering in some manner its existence sensi- 

 ble. The first would require the observer tcH^e possessed of a prism and 

 a lens of transparent rock salt. Although recent investigations had shown, 

 as had been anticipated, that this substance was not perfectly transparent 

 with respect to dark radiant heat, yet, of all solid or liquid substances 

 hitherto examined, it w^as by far the most nearly transparent. The most 

 hopeful direction in which to look for ready means of rendering sensible 

 the presence of rays of low refrangibility seemed to be their chemical 

 effects. If some of those who were skilled in photography would turn 

 their attention in this direction, we might seen be in poie&icn of very 



