. Prof. Powell's 'Notes on Repulsion by Heat^ 8^c. 3 1 9 



on the polarized tints developed by heated glass, it appears 

 that the heat is transmitted instantaneously through the glass, 

 the system of colours appearing complete at once. This would 

 show that in point of fact no warping at all takes place : the 

 two opposite surfaces are simultaneously heated. 



(2.) I made some trials for ascertaining the time required 

 for the communication of heat through two glasses, when at 

 different small intervals of separation, as marked by the ap- 

 pearance of different tints of the thin plates. A delicate mer- 

 curial thermometer was placed with its bulb resting on the 

 upper plate exactly at the point where the central tint was 

 formed. Heat was applied below by the flame of a spirit- 

 lamp playing against the under side of an iron plate on which 

 the glasses rested. The time was noted on which the ther- 

 mometer rose 1° Centigrade under the different circum- 

 stances ; the results were as follows : 



(3.) To compare the transmission of heat when the glasses 

 were kept in close contact by forcible compression by means 

 of a wedge, &c., and when loose, the following results were 

 observed with a thermometer touching the upper glass : 



Time of rising 1** Centig. 

 Glasses compressed ... ..... 30 sec^ 



Glasses loose 40 sec^. 



(4.) In these cases part of the heat is employed in separa- 

 ting the glasses by repulsion, part communicated through 

 them. It would form an interesting subject of inquiry to ex- 

 amine the law by which this is regulated. Many questions 

 naturally arise as to the connection of such results with the 

 communication of heat and expansion in general, — and thus 

 with the specific heat of bodies, and their expansion in dif- 

 ferent directions, especially in crystals. Perhaps some results 

 might be obtained by comparing the transmission of heat 

 through crystals in the direction of cleavage and in that at 

 right angles to it, especially those which split into laminae, 

 as sulphate of lime, &c. 



(5.) When a large central black spot is formed, the attrac- 

 tion is so great that it seems impossible to overcome it by the 

 repulsion of the heat. In one experiment a large black spot 

 was formed, and heat being continued for 3"^ 30% no change 

 had taken place, when the upper glass cracked, but not so 



