CHEMICAL DECOMPOSITION. 97 



separated farther from each other : by continuing- this 

 influence, after a certain time the cohesion of the mass is 

 so reduced that it melts, or becomes liquid, and, under 

 the force of a still higher temperature, this molten metal 

 may be dissipated in vapour. It would appear as if, 

 under the agency of the heat applied to a body, its atoms 

 expanded, until at last, owing to the tenuity of the outer 

 layer or envelope of each atom, they were enabled to 

 move freely over each other, or to interpenetrate without 

 difficulty. That heat does really occasion a considerable 

 disturbance in the corpuscular arrangement of bodies, 

 may be proved by a very interesting experiment. A 

 bar of heated metal is placed to cool, with one end sup- 

 ported upon a wedge or a ring of a different metal the 

 Other resting on the ground. In cooling, a distinct 

 musical sound is given out, owing to the vibratory action 

 set up among the particles of matter moving as the tem- 

 perature declines.* 



-> This curious phenomenon was first observed by Mr. Trevelvan, 

 whose Notice regarding some Experiments on the Vibration of Heated 

 Metals will be found in tbe Transactions of the Koyal Society of 

 Edinburgh, vol. xii., 1837. In a Memoir in the same volume, 

 entitled Experimental Researches regarding certain vibrations whidi 

 take place between metallic masses having different temperatures, 

 Professor Forbes draws the following conclusions : 



1. " The vibrations never take place between substances of the 

 same nature. 



2. " Both substances must be metallic. (This is now proved 

 not to be necessary.) 



3. " The vibrations take place with an intensity proportional 

 (within certain limits) to the difference of the conducting power- 

 of the metals for heat or electricity ; the metal having the least 

 conducting power being necessarily the coldest. 



4. " The time of contact of two points of the metals must be 

 longer than that of the intermediate portions, 



5. " The impulse is received by a distinct and separate pro- 

 cess at each contact of the bar and block, and in no case is the 

 metallic connection of the bearing points in the bar, or those of 

 the block, in any way essential. 



6. "The intensity of the vibration is (under certain ex 



H 



