32 CORRELATION OF PHYSICAL FORCES. 



us by the ticking sound. But it will be said, if instead of 

 allowing the weight to act upon the machinery, the cord by 

 which it is suspended be cut, the weight drops and the force 

 is at an end. By no means, for in this case the house is 

 shaken by the concussion, and thus the force and motion are 

 continued, while in the former case the weight reaches the 

 ground quietly, and no evidence of force or motion is mani- 

 fested by its impact, the whole having been previously dissi- 

 pated. 



If the initial motion, instead of being arrested by the im- 

 pact of other bodies, as in friction or percussion, is impeded 

 by confinement or compression, as where the dilatation of a 

 gas is prevented by mechanical means, heat equally results : 

 thus if a piston is used to compress air in a closed vessel, the 

 compressed air and, from it, the sides of the vessel will be 

 heated : the air being unable to take up and carry on the 

 original motion communicates molecular motion or expansion 

 to all bodies in contact with it ; and, conversely, if we ex- 

 pand air by mechanical motion, as by withdrawing the pis- 

 ton, cold is produced. So when a solid has its particles com- 

 pressed or brought nearer together, as when a bar of iron is 

 hammered, heat is produced beyond that which is due to per- 

 cussion alone. In this latter case we cannot very easily ef- 

 fect the converse result, or produce cold by the mechanical 

 dilatation of a solid, though the phenomena of solution, 

 where the particles of a solid are detached from each other, 

 or drawn more widely asunder, give us an approximation to 

 it : in the case of solution cold is produced. 



We are from a very extensive range of observation and 

 experiment entitled to conclude that, with some curious ex- 

 ceptions to be presently noticed, whenever a body is com- 

 pressed or brought into smaller dimensions it is heated, i. e. 

 it expands neighbouring substances. Whenever it is dilated 

 or increased in volume it is cooled, or contracts neighbouring 

 substances. 



