514 



to what M. Hermite has called the "four types" of binary 

 quintics. 



Other covariants of the cubic are discussed, and in particular a 

 general expression is given for that covariant of the 9th order which 

 geometrically represents a surface passing through the twenty-seven 

 right lines on the surface of the third order represented by the cubic. 



III. " On the Construction of a new Calorimeter for determining 

 the Radiating Powers of Surfaces, and its application to the 

 Surfaces of various Mineral Substances." By W. HOPKINS, 

 Esq., M.A., F.R.S., &c. Received June 1, 1860. 



(Abstract.) 



When the author's Memoir on the Conductivity of various sub- 

 stances was presented to the Society, it was intimated to him on the 

 part of the Council of the Society, that it might be advisable to de- 

 termine absolute instead of relative conductivities, the latter only 

 having been attempted in his previous experiments. It is partly 

 in consequence of this intimation, and partly from the desire to make 

 his former investigations more complete, that the author has given 

 his attention to the construction of a calorimeter which might serve 

 for this purpose. His present memoir contains a description of this 

 instrument, with the results obtained from its application to the 

 surfaces of various substances. 



The apparatus used by Messrs. Dulong and Petit was more deli- 

 cate and complete than the simpler instrument devised by the author 

 of this paper, but it was calculated only to determine the radiating 

 powers of substances of which the bulb of a thermometer could be 

 constructed, or with which it could be delicately coated. The only 

 substances to which, in fact, it was applied, were glass and silver, 

 the radiation taking place, in the first case, from the naked bulb of 

 the thermometer, and, in the second, from the same bulb coated 

 with silver paper. In these cases, too, it was the whole heat radi- 

 ating in a given time from the instrument, and not that which radiated 

 from a given area, that was determined. For this latter purpose the 

 apparatus was not well calculated, on account of the difficulty of ob- 

 taining with accuracy the area of the surface from which radiation 

 took place. The instrument here described can be easily applied to 



