FORCES AND ORGANS. 9 



which it embraces ; that of the unity of the physical forces 

 tends to absorb them all. From the invisible atom to the celes- 

 tial body lost in space, everything- is subject to motion. Every- 

 thing gravitates in an immense or in an infinitely little orbit. 

 Kept at a definite distance one from the other, in proportion 

 to the motion which animates them, the molecules present 

 constant relations, which they lose only by the addition or the 

 subtraction of a certain quantity of motion. In general, 

 increase of motion enlarges the orbit of the molecules, and 

 widening their distance from each other, increases the volume 

 of the bodies. By this rule, heat is proved to be a 

 source of motion. Under its influence the molecules, becom- 

 ing more and more separated, cause bodies to pass from 

 solid to liquid, and then to a gaseous state. These 

 gases become indefinitely dilated by the addition of fresh 

 quantities of heat. But that force which lends extreme 

 rapidity to the motion of the molecules, that force which is 

 admitted in theory is rendered tangible by experiment ; its 

 intensity is measured by opposing to the dilatation of a body 

 an obstacle which it will have to surmount. Thus it is that 

 the molecules of gases or vapours imprisoned in the cylinder 

 of machines, communicate to the partitions and to the piston 

 the pressure which is employed in producing action by 

 machinery. This mechanical action is, in its turn, trans- 

 formed into heat if the conditions of the experiment be re- 

 versed ; if, for example, an external force, thrusting back 

 the piston of an air-pump, restrains the molecular motions by 

 violent compression. 



The new theory has thrown light upon certain hypotheses, 

 those, among others, which claimed admission for the latent 

 heat of fusion, or of vaporisation of bodies, the latent heat of 

 dilatation of gases. It has suppressed others ; for instance, 

 the discovery of atmospheric pressure has banished the 

 hypothesis which has now become ridiculous, that nature 

 abhors a vacuum. 



Although the theory accommodates itself with less ease to 

 the interpretations of luminous and electric phenomena, it 

 admits, according to the great analogy between these phe- 

 nomena, and heat, of supposing that they themselves are only 

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