Dr. Woods on the Heat of Chemical Combination. 45 



body is always necessary ; and hence that we might altogether 

 dispense with the idea of attractions and repulsions between the 



particles of matter. 



(16.) But if these "forces" do not exist, if the particles of 

 matter are passive with respect to each other, what is their bond 

 of union ? what makes them cohere ? A little consideration 

 shows us that no change occurs in one direction only ; no change 

 in the conditions of matter or motion takes place without an 

 opposite one being produced at the same time ; hence what is 

 called cause and effect, action and reaction, &c, one being always 

 accompanied by the other. However disguised its opposite may 

 be, still for every change an opposite must exist. Grove's "cor- 

 relation of forces," although not intended to demonstrate this 

 principle, offers many instances of it. We cannot set a body in 

 motion without producing rest or opposite motion in some other 

 body ; we cannot elevate a weight without depressing the earth ; 

 we cannot warm one body without cooling some other. No matter 

 what is done, reflection will bring us to admit that some opposite 

 change takes place ; that, in fact, to produce any result, a cer- 

 tain power must be employed, and this power when analysed is 

 equal and opposite to the work done. Now if we apply this first 

 principle to the expansion and contraction of bodies, what need 

 have we of any such powers as attractions and repulsions between 

 the particles ? If one body expand, some other should contract ; 

 motion in one direction requiring motion in the reverse. Joining 

 this truth therefore with what is palpable, that for all tempera- 

 tures bodies have a certain fixed volume, or that a relation exists 

 between matter and the space it occupies, and consequently a 

 relation between the volumes of different substances, we see that 

 it is of no consequence how devoid of action the particles of the 

 same bodies may be on each other. Not only they must cohere, 

 but at first sight it would appear impossible to produce expan- 

 sion or contraction at all ; for to compress or expand a piece of 

 iron for instance, not only must an opposite effect be produced 

 in some other body, but all other matter reacts to keep up the 

 relation which should exist between the volumes. But we also 

 see that if by any means we could produce contraction, a corre- 

 sponding expansion must result among other bodies. 



(17.) As I said above, I believe the only argument in favour of 



' radiation of heat into space, or contraction without corresponding 

 expansion, is that the earth is said to radiate. We have unfor- 

 tunately no means of testing its truth experimentally. We have 

 however an indirect method of proving that bodies do not part 

 with volume without an accompanying increase in some other ; 

 for let a gas, suppose, or any other substance, be made to con- 

 tract by pressure, however small this contraction may be, we 



