PHILOSOPHICAL SOCIETY OP WASHINGTON. 81 



The minute wave-motions which result from atomic periods 

 have, as we all know, a far wider range than that recognized by 

 sight; and these wave motions are capable, as all know, of exert- 

 ing a dynamic effect which may be indefinitely accumulated. But 

 there is no accumulation of "light." Hence, to speak of the 

 "mechanical equivalent" of light, or of determining the pressure 

 of a luminous ray, or of "weighing sunshine," is simply to misuse 

 words, and to confound incongruous ideas. 



As to the interpretation of this curious and undoubtedly ab- 

 struse phenomenon, Mr. Taylor thought it to be tolerably well 

 established that neither attenuated air-currents, nor evaporation, 

 nor "electricity" had any agency in the case; but that the ex- 

 planation given by Prof. Dewar, of Edinburgh, and by Mr. 

 Stoney, was undoubtedly the true one, namely, that the blackened 

 side of the disks absorbing more heat-motion than the unblack- 

 ened side, communicated more reactionary motion to the ex- 

 tremely rarefied air in the small glass chamber. Mr. Crookes 

 undoubtedly deserved great credit for the ingenuity and patience 

 displayed in varying and testing the experiments. The name 

 "radiometer," however, he thought unhappily chosen, as it sug- 

 gested or countenanced the idea that the vanes were acted upon 

 by the impulsion of radiation; whereas the instrument was really 

 a differential absorption-scope. 



Mr. S. Newcomb suggested that this part of the question could 

 very easily be tested by arresting the motion of the vanes (either 

 by a strong magnet or otherwise), throwing a strong beam of 

 radiant heat upon the blackened disk, then suddenly cutting off 

 entirely the source of radiation and removing the detent, to ob- 

 serve whether any motion occurred. If it should, then evidently 

 this could not be the effect of direct impulse or momentum from 

 the radiation. 



As to the explanation alluded to by Mr. Taylor, the speaker 

 understood that Mr. Stoney's theory, as set forth in the Philo- 

 sophical Magazine for March and April last, involved a reaction 

 between the heated disk and the side of the glass chamber, through 

 the medium of the residual air; this reaction commencing when 

 the air becomes so rare that the molecules rebounding from the 

 surface of the vane reach the enclosing chamber without raeetino- 

 other molecules. If this explanation be correct, it would follow 



