694 THE POPULAR SCIENCE MONTHLY. 



light entering his room is likely to injure the sensitive surfaces there 

 exposed ; thus, having ascertained by experience that his plates are 

 fogged, or his paper injured, when the revolutions exceed, say, ten a 

 minute, he will take care to draw down an extra blind when the 

 revolutions approach that number. Still more useful will the radi- 

 ometer be in the photographic gallery. Placing an instrument near 

 the sitter at the commencement of the day's operations, it is found 

 that, to obtain a good negative, the lens must be uncovered not for 

 a particular number of seconds but during the time required for the 

 radiometer to make, say, twenty revolutions. For the remainder of 

 the day, therefore, assuming his chemicals not to vary, the operator 

 need not trouble himself about the variation of light ; all he has to do 

 is to watch the radiometer and expose for twenty revolutions, and his 

 negatives will be of the same quality,' although at one time it may 

 have taken five minutes, and at another not ten seconds, to perform 

 the allotted number. 



I have long been experimenting in the endeavor to trace some con- 

 nection between the movements of attraction and repulsion above 

 alluded to and the action of gravitation in Cavendish's celebrated 

 experiment. The investigation is not sufficiently advanced to justify 

 further details, but I will give here an outline of one of the results. 



I find that a heavy metallic mass, when brought near a delicately- 

 suspended light ball, attracts or repels it under the following circum- 

 stances : 



I. When the hall is in air of ordmary density. 



a. If the mass is colder than the ball, it repels the ball. 

 h. If the mass is hotter than the ball, it attracts the ball. 



II. When the hall is in a vacuum, 



a. If the mass is colder than the ball, it attracts the ball. 

 h. If the mass is hotter than the ball, it repels the ball. 



The density of the medium surrounding the ball, the material of 

 which the ball is made, and a very slight difference between the tem- 

 peratures of the mass and the ball, exert so strong an influence over 

 the attractive and repulsive force, and it has been so difficult for me 

 to eliminate all interfering actions of temperature, electricity, etc., 

 that I have not yet been able to get distinct evidence of an indepen- 

 dent force (not being of the nature of heat or light) urging the ball 

 and the mass together. 



Experiment has, however, shown me that, while the action is in 

 one direction in dense air, and in the opposite direction in a vacuum, 

 there is (as I have already pointed out in the experiments described 

 in the commencement of this paper) an intermediate pressure at which 

 differences of temperature appear to exert little or no interfering ac- 



' In this brief sketch I omit reference to the occasions in which the ultra-violet rays 

 diminish in a greater proportion than the other rays. 



