RADIANT HEAT. 349 



Repulsive power of Heat: Powell. 



Closely connected with the radiation of heat is its property of exert- 

 ing or exciting a repulsive force between particles or masses of mat- 

 ter at small though sensible distances. 



Such a property was first announced by Libri in 1824; and was 

 further examined by Fresnel {Ann. de Chim., xxix, 57, 107) and Saigcy, 

 [Bull. Meth., xi, 1G7;) 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 Professor 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 vibrations 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 decisive proof of the fact of 

 repulsion. 



The essential principle is the employment of the colors 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 partially discussed by Mr. 

 Knight in the Philosophical Transactions, 1816. Mr. MacKeevor and 

 and Mr. Eisdale subsequently investigated the theory, and M. Arago 

 gave a discussion of the whole question in the Annuaire, 1833, and 

 in the Edinburgh New Philosophical Journal, vol. xv, p. 128; lastly, 

 a highly curious paper appeared in the Philosophical Transactions for 

 1835, Part II, "On the ice formed under peculiar circumstances at 

 the bottom of running water," by the Rev. J. Farquharson, F. R. S., 

 of Alford, Aberdeenshire. In this paper the author details various 

 new and highly interesting particulars as to the mode of the formation 

 of the spongy masses of spiculse of ice at the bottom of certain rivers 

 in his neighborhood, and the peculiar circumstances under which 

 alone it is formed. He examines acutely the several explanations 

 which have been suggested, which he shows are all insufficient to 

 explaiu the whole of the circumstances, and then proceeds to suggest 



* See Report, 1834, p. 549, and Dr. Thomson's Records of Science. 



