Dr. Woods on the Heat of Chemical Combination. 43 



diminution in the elasticity of the condensed volume of gas ; 

 therefore the gaseous particles, after having undergone reflexion 

 from the walls of the tube, possess a lower velocity than that 

 which they previously had. 



Dr. Faraday's theory may perhaps be regarded as a particular 

 instance of this more general theory of abrasion. On bringing 

 the two surfaces together, certain particles which are most favour- 

 ably circumstanced may discharge to each other. No frictional 

 electricity is produced by this operation ; but the particles are 

 united by cohesion, and could remain thus combined for an inde- 

 finite period without interfering with the subsequent process, 

 which consists in the rending of these two sets of particles from 

 each other, and which, as in other instances of permanent abra- 

 sion, developes the electricity. The following appears to be an 

 example of this process, and there are plenty of the same sort. 

 Place melted sealing-wax on a glass plate — this brings the two 

 surfaces so near together that they discharge and cohere; on 

 rending the sealing-wax from the glass, the electricity is de- 

 veloped. 



The production of hail cannot generally be unaccompanied 

 with the development of electricity. We know from Dr. Waller's 

 observations, that hail at one period of its formation consists of 

 conglomerations of snow mingled with water. As long, there- 

 fore, as the surface remains moist, the particles of water can be 

 torn away by the wind ; and it is even probable, from the solidity 

 of the mass, that the abrasion from the surface of a partially 

 frozen hail-stone may be even greater than if the whole mass 

 was water at the same temperature. 



7 Prospect Place, Ball's Pond Road, 

 November 19, 1851. 



VII. On the Heat of Chemical Combination. 

 By Thomas Woods, M.D. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen Parsoiistown, Dec. 1851. 



(13.) TTAVING proved experimentally in my paper in this 

 Al Magazine of last October, " that the decomposition 

 of a compound body occasions as much cold as the combination of 

 its elements originally produced heat," I will now premise my 

 theory of the cause of the heat of chemical combination by a few 

 remarks on the Molkcui.ak Constitution of Matter. 



If we take two equal cubes of any substance, iron for example, 

 one bested to 1000° C, the other at 0°, and place them together, 

 we find that the former contracts and the laid r expands until 



