PHYSICS. 233 



the conductivities themselves, but to a somewhat higher 

 power of them. 



Buff has experimented to determine the thermal conduc- 

 tivity and diathermancy of air and hydrogen, using for this 

 purpose an apparatus similar to that used for the same pur- 

 pose by Magnus, but modified considerably in its details. 

 From the results obtained he concludes that the thermal 

 conductivity of hydrogen and of other gases is far too small 

 to admit of its being proved by the method Magnus adopted, 

 the assumption sometimes made that hydrogen conducts 

 heat like a metal not being justified; that, on the other hand, 

 hydrogen possesses a diathermancy closely approaching that 

 of a vacuum; that dry air absorbs from fifty to sixty per 

 cent, of the rays of heat which it receives from a source 

 heated to the boiling-point of water; that the absorptive 

 power of moist air surpasses that of dry air by a trifling 

 percentage, but by no means to such a degree as hitherto 

 had been assumed by several physicists ; and that rock-salt 

 is not absolutely diatliermanous for the so-called dark rays 

 of heat ; its thermal color rather resembles that of dry air 

 in this respect. 



Stokes has described the results of some experiments made 

 witli a radiometer whose vanes were all metallic, one side of 

 each being roughened by ruling it closely with a sharp knife. 

 In every case it appeared that when the fly is hotter than 

 the bulb, the rough surface is repelled, and, when cooler, at- 

 tracted. Results nearly the same were obtained with anoth- 

 er radiometer, one side of each vane of which was electro- 

 coated with finely divided silver. Hence Professor Stokes 

 concludes that there are three conditions under which mo- 

 tion may be obtained in a radiometer: 1st, difference of 

 temperature on the two faces, as in a pith radiometer, coat- 

 ed on one face with lamp-black; 2d, more favorable pres- 

 entation of one face than the other, as in a radiometer with 

 carved disks; and, 3d, roughness of surface on one face 

 if this be really different from the last. These effects, it is 

 obvious, may be variously combined so as to oppose or as- 

 sist each other in producing motion. 



3. Specific Heat and Thermodynamics. 



Lecher has communicated to the Vienna Academy the im- 



