20 REPORT— 1840. 
It thus distinctly appears, that through so extensive and 
varied a range of differences in the state of the radiating sur- 
face, no determinate relation subsists between the radiating 
power, end either darkness of colowr, or any other distinctive 
character of the coating employed; not even its roughness or 
smoothness. 
Repulsive Power of Heat: Powell. 
Closely connected with the radiation of heat is its property of 
exerting or exciting a repulsive force between particles or masses 
of matter at small though sensible distances. 
Such a property was first announced by Libri in 1824; and 
was further examined by Fresnel (dun. de Chim., xxix. 57. 
107.) and Saigey (Bull. Meth., xi. 167.), but their results seem 
to have been open to some doubt. 
A new interest attached to the subject from the reference 
made to this property by Prof. Forbes (in a paper read to the 
Royal Society of Edinburgh, March,1833, and since published 
in their Transactions, vol. xii.) in explanation of certain vibra- 
tions of heated metals first observed by Mr. Trevelyan. 
A paper from me was read to the Royal Society, June 19,1834, 
and printed in the Philosophical Transactions, 1834, Part II. 
containing an account of experiments on a different principle 
from any of the preceding, which appeared to furnish a deci- 
sive proof of the fact of repulsion. 
The essential principle is the employment of the colours of 
thin plates, as a measure of the separation produced between two 
surfaces, by the repulsive action of heat applied to one of them. 
I also made observations on several particulars attending the 
mode of action, both in that paper, and in a communication to 
the British Association at the Edinburgh meeting*. 
Formation of Ice: Farquharson. 
An interesting case, in which the principles of the theory of 
radiant heat are related to the explanation of natural phenomena, 
occurs in the instance of the formation of ice exclusively at the 
surface of still water, but occasionally at the bottom of running 
water. This point excited attention some years ago, and was par- 
tially discussed by Mr. Knight in the Philosophical Transactions, 
1816. Mr. MacKeevor and Mr. Kisdale subsequently investi- 
gated the theory, and M. Arago gave a discussion of the whole 
question in the Annuaire, 1833, and in the Edinburgh New Phi- 
losophical Journal, vol. xv. p. 128 ; lastly, a highly curious paper 
appeared in the Philosophical Transactions for 1835, Part II. 
* See Report, 1834, p. 549, and Dr. Thomson’s Records of Science. 
