52 Dr. Woods on the Heat of Chemical Combination. 



action, viz. that affinity depends on external circumstances. It 

 would be needless to quote the many instances by which this 

 idea is verified — instances in which the power to combine between 

 substances is altogether destroyed or heightened, according to 

 the circumstances in which they are placed. It will be found 

 that in proportion as the two opposite effects are provided for, 

 so will bodies combine, just as condensation or expansion can be 

 accomplished, as the reverse is made easy. The objection that, 

 because chemical action changes the properties of bodies, it can- 

 not be a mere mechanical one, does not, I think, hold good. An 

 acid and alkali combining neutralize each other's previous effect, 

 and an innocuous compound results ; but their properties are not 

 changed. If we take a certain quantity of steam and of ice, one 

 will scald, the other freeze — one is solid, the other gaseous. 

 When mixed, the compound is innocuous, and neither solid nor 

 gaseous ; yet it must be admitted that it is a mechanical mixture. 



(30.) I would remark, before summing up, that not only does 

 the approximation of diverse particles, as in chemical combina- 

 tion, assimilate in the effect it produces, the contraction of like 

 particles in the same body, but that, as we noticed in (24.), the 

 amount of expansion or contraction in certain cases depended on 

 one of the elements only, so in chemical combinations one ele- 

 ment determines the amount of heat or expansion also; for 

 instance, oxygen uniting with several combustibles gives the 

 same amount of heat ; the same base always produces the same 

 quantity of heat, no matter how different the body may be with 

 which it unites ; these analogies between the conduct of the par- 

 ticles of the same body, and that of particles of a diverse nature, 

 adding to the proof that the theory I advance to account for the 

 heat of chemical combination is the true one. 



(31.) I have endeavoured to condense as much as possible into 

 a reasonable limit, but I am afraid I have taken up too much 

 space, I will therefore sum up briefly what I think I have proved. 



That there is a balance between the distances of the particles 

 of all matter, these distances being different for different bodies. 



That to preserve this balance, motion among the particles of 

 one body cannot be effected without a relatively equal and oppo- 

 site one in some other. 



That in contractions and expansions, as the volume gained or 

 lost has a relation to the volume already possessed by the body, 

 an extensive movement among particles far apart compensates a 

 small one among those which are close together. 



That, therefore, when particles, though of an opposite nature, 

 come together in chemical combination, the particles of oilier 

 bodies must expand to supply the opposite effect ; and when 

 these chemically combined particles separate in decomposition, 

 a contrary movement, or contraction, or cold is produced. 



