486 



PROFESSOR POWELL ON THE REPULSIVE POWER OF HEAT. 



rally diminished, with thicker discs. He mentions other points on which his results 

 had not been equally decisive, but allows that the whole subject requires further ex- 

 amination. The completion of this interesting inquiry is doubtless one of the nu- 

 merous benefits of which science was deprived by his early loss. 



M. Saigey*, in the course of a series of experiments on the development of mag- 

 netism in certain metallic bodies, notices some effects of repulsion, which (after exa- 

 mining every ordinary cause likely to have occasioned them,) he concludes by refer- 

 ring to heat. He tried the effect by means of a needle of lead finely suspended at 

 different distances from a bar of copper, and found the number of oscillations in a 

 given time decrease with the distance ; that is, the needle more rapidly assumed the 

 position of parallelism to the heated bar in which repulsion would tend to place it. 



Signor Libri's result is remarkable as contradicting the statement of Laplace -|-, 

 who speaks of the " repulsive force of heat" as subsisting among the particles of a 

 fluid ; but observes that experiment shows it has no other effect on capillary attrac- 

 tion than what results from its diminishing the density of the fluid. 



In trying to repeat Libri's experiment, I have never been able to succeed, except 

 in producing a slight apparent motion in the drop, which seems explicable from the 

 mere effect of evaporation on the side next the heat. 



I have observed a drop of oil, contained in a glass tube of about one tenth of an 

 inch bore, move away from the part where heat is applied, evidently from the expan- 

 sion of the glass, which renders the tube slightly conical, when the drop moves 

 towards the narrower end. I have applied heat to capillary tubes till the suspended 

 liquid has boiled, without producing any effect ; to inclined glasses between which a 

 drop of oil was advancing, without in the least affecting its motion ; and to a plate 

 of glass from the under side of which a globule of -mercury remained suspended, 

 without overcoming the attraction. 



With regard to repulsion at greater distances, on employing an arrangement some- 

 what similar to Fresnel's, when the discs were two small plates of glass with truly 

 plane surfaces, I found that if in the first instance they were pressed together, so as 

 to adhere, heat always overcame the attraction, and the moveable disc sometimes 

 receded to a sensible distance. But this effect (and perhaps also that in Fresnel's 

 experiment) appeared to me 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 ex- 

 pansion of the more heated surface, producing a convexity towards the heat. 



The amount to which this takes place may be easily calculated from the known 

 dilatation of glass, the difference of temperatures of the two surfaces, and the thick- 

 ness. 



In some cases, the two glasses were pressed so hard together that the colours of 

 thin plates appeared between them. On the application of heat, these colours in- 



* Bulletin Mathematique, torn, xi. No. 167. 

 t M^canique Celeste, Supp., livr. x. p. 75. 



