504 
ME. W. CEOOKES ON ATTE ACTION AND 
original position. To see if these phenomena were due to the residual gas, air was 
gradually let in ; and on repeating the experiment when the density of the enclosed air 
was fifteen or twenty times greater than at first, it was found that the repulsion had 
not sensibly increased, as it should have done had it been due to currents of heated air. 
Under some conditions, indeed, the movement was not so great as in a vacuum. Some- 
times Fresnel observed an action of attraction, the disks adhering when heated, and 
separating when the lens was removed. With pieces of copper suspended to the mag- 
netic needle the attraction was very apparent ; when the movable and fixed disks were 
near together they approached on applying heat. Reasons are given why these effects 
cannot be due to electricity or magnetism, but the author does not seem to have tried 
any further experiments. 
8. M. Saigey in 1827 * described an experiment with a needle of lead delicately 
suspended at different distances from a bar of copper. He found the number of oscil- 
lations in a given time decrease with the distance. From his experiments he arrived 
at the following results : — All bodies exert between themselves a feeble repulsive action 
under ordinary circumstances. A very marked attraction may be observed between a cold 
and a heated body, or between two bodies of different temperatures, whether screens be 
interposed or not. Saigey concludes that in many cases results obtained without the 
appreciable development of magnetism or electricity have been attributed to these 
forces. 
9. In the year 1834 Professor J. D. Forbes f published an elaborate research on the 
vibrations which Mr. A. Trevelyan had found to take place between metallic masses 
having different temperatures. The general conclusion at which he arrived is that 
there is a repulsive action exercised in the transmission of heat from one body into 
another which has a less power of conducting it. These repulsions only take place 
between bodies having a certain amount of conducting-power, below which some metals 
fall ; it must be excitable in a most minute space of time, and is energetic in propor- 
tion to the difference of conducting-power of the substances and to their difference of 
temperature. 
10. The Rev. Baden Powell, in the same year J, published a paper “ On the Repul- 
sive Power of Heat.” He employed an arrangement somewhat similar to Fresnel’s (7), 
the disks being two small plates of glass with truly plane surfaces. He found that if 
in the first instance they were pressed together so as to adhere, heat always overcame 
the attraction, and the movable disks sometimes receded to a sensible distance ; but 
Professor Powell says that this effect (and perhaps also that in Fresnel’s experiment) 
appeared to him in a great measure due to another cause than repulsion, viz. the slight 
curvature which will be given to the plate of glass by the greater expansion of the more 
heated surface producing a convexity towards the heat. 
11. By pressing the disks closely together, the coloured rings formed would give a 
* Bulletin Mathematique, tom. ix. pp. 89, 167, 239, tom. xi. no. 167 ; Bull. Sci. Nat. viii. p. 287. 
t Trans. Eoy. Soc. Edinburgh, vol. xii. p. 429. i Philosophical Transactions, 1834, p. 485. 
