416 Mr. Addams on the Repulsive Actioji of Heat. 



tion between the metal and silica was so trifling that the latter 

 would often remain stationary, whilst the former slid beneath 

 it ; and when the capsule was moved, by the hand, circularly 

 in one direction, the powder would revolve the contrary way. 

 The effect ceases almost instantly by a removal from the lamp, 

 and is renewed as often as it is reheated. Whilst subjected to 

 the flame of the lamp, if touched with a cold body, as a metallic 

 wire or glass rod, it would sometimes lose its peculiar freedom 

 to glide about, and then rest upon the platinum vessel slug- 

 gish and immoveable otherwise than with the vessel itself. 



Exp. 2. Supposing it possible that moisture might inter- 

 fere, I introduced another portion of silica into a glass flask, 

 mounted with a stop- cock, and heated it to a temperature of 

 from 300° to 500°, in which state it was kept for a week with 

 the valve closed. After the first action of the heat to expel a 

 part of the air, it still continued to exhibit the same phaeno- 

 menon as before in the open vessel. The air was additionally 

 rarified by an air-pump, but with no alteration in the beha- 

 vour of the powder. 



Exp. 3. Magnesia, peroxide of copper, sesquioxide of lead, 

 peroxide of cobalt, oxide of nickel, peroxide of manganese, 

 and smalt, were successively heated as in Exp. 1, and with 

 correspondent results. 



On the other hand, oxide of chromium, litharge, and alu- 

 mina afforded little or no evidence of such peculiarity. 



Remembering Dr. Faraday's experiment " on the electric 

 powers of oxalate of lime*," I subjected that compound to a 

 similar trial ; and obtained evidence of the same kind of mo- 

 tion as before (1.), yet in a less degree than with silica and 

 the substances named in No. 3. from magnesia to smalt in- 

 clusive. 



The powders were more or less in an electric state, but this 

 (the electricity,) I regard as an accompaniment, and not as the 

 cause of the free motion of the powders upon the metal ; for 

 touching the metal with good conductors of electricity made 

 no alteration save that which could fairly be assigned to the 

 cooling effect upon the platinum or glass capsules, both 

 having been employed. Also by insulating the platinum no 

 difference was noticed. 



In Dr. Faraday's experiment before referred to, the ox- 

 alate of lime was electrified, positively, by stirring it with a 

 rod or spatula, whereas the diminished friction or contact be- 

 tween the bodies concerned in the experiments now de- 

 scribed took place without stirring or previous agitation. 



* Journal of the Royal Institution, vol. xix. p. 338. 



