Dr. Woods on Chemical Combinatiun. 373 



change at all would have taken place. For example : if potassium 

 or zinc be placed in Avater, the metal^ the oxygen and the hy- 

 drogen are at an insensible distance from each other ; but as the 

 distance required between oxygen and zinc is less than between 

 any other two of the elements, or between the particles of either 

 of the elements by itself, the oxygen and zinc unite or come 

 together, while the opposite movement or expansion is supplied 

 partly by the expansion or separation of the oxygen and hydrogen^ 

 and partly by external bodies. Zinc can therefore decompose 

 water ; but if copper be placed in water, as it does not produce 

 as much heat by combining with oxygen as hydrogen does, or in 

 other words, does not lie so closely to it in combination, no 

 change is produced ; the water is not decomposed. If, however, 

 oxide of silver in solution be substituted for the water, as copper 

 produces more heat in combining witho xygen than silver does, 

 the oxide of silver is decomposed, the oxygen uniting witli the 

 copper. Or if nitric acid and copper be mixed, as the last pro- 

 portions of oxygen do not produce so much heat with the 

 nitrogen as they do with the copper, the nitric acid is decom- 

 posed and oxide of copper formed. 



Thus the heat produced is a measure of the so-called affinity 

 of bodies ; for heat is but expansion among particles ; and when- 

 ever expansion occurs, contraction must be simultaneously going 

 on ; that contraction in chemical action is a lessening of the 

 distances between the combining particles; and the greater this 

 is, the greater is the heat. 



The movements occurring in chemical combination are thus 

 referred to the same causes, and made to differ in nothing from 

 those occurring in simple matter when it contracts or expands. 

 Perhaps the phfeuomena of what is called " latent heat " may 

 more clearly express my meaning. I consider the particles of 

 combining bodies to be similarly circumstanced with those of 

 steam becoming condensed ; the distance between the particles 

 in either case 1)ecomes less ; the opposite effect, or expansion, or 

 heat in the case of steam, is called the latent heat ; in the case 

 of the chemical action it is called the heat of chemical combina- 

 tion. They differ from each other in nothing exce})t amount; 

 and this I endeavoured to account for in the January Number 

 of the Pliilosophical Magazine, 1852. 



To acceunt, then, for chemical action, and the heat pi'oduced 

 by it, we have only to admit the existence of two laws : — 



1st. Tliat the distance between the particles of bodies has 

 some dependence on the matter composing them. 



2iid. That any change in this distance in one is necessarily 

 accompanied l)y an equal and c)])posite change in another. 



As a postscript, I would say that the idea of attractions and 



