AT THE MOMENT OF CHEMICAL CHANGE. 
763 
taneous, but correlative and mutually dependent phenomena which we cannot sepa- 
rate. 
In the case of double 'decomposition, each of the four substances which enter into 
the change is combined, but it does not appear that this state of combination is 
necessary to the action. It can take place also, and in the same manner, when the 
combining substances are only in contact with each other, and not in combination, 
provided always that there is the right chemical difference between them, which 
however is essential. Thus, for example, when iodine and phosphorus decompose 
water (in the usual mode of the formation of hydriodic acid), the chemical relation 
between the iodine and phosphorus is an essential condition of the action. The same 
remark applies to the decomposition of water between nitric oxide and chlorine, 
which can be effected by neither body separately ; so that the changes which take 
place in these experiments are not simply due to the fact that the chlorine or iodine 
stand in one relation to water, or to the elements of water, and the nitric oxide or 
phosphorus in another, and that thus the water breaks up, being acted upon by two 
opposite forces ; but that there is also, and must be, a certain chemical difference 
between the chlorine and nitric oxide and between the iodine and phosphorus, which 
is as essential and important a condition to the propagation of the action as their re- 
lation to the water itself, and indeed without which they could not have this relation. 
I am not aware that this remark has before been made, nor do I think it likely that 
it should have been made, but upon the view which I have given, of which it is a 
consequence. 
Facts corresponding to these cases of composition are to be observed, as might be 
expected, in the decomposition of bodies, for if decomposition be the condition of 
combination, so of course must combination be the condition of decomposition. 
Faraday long since showed that dry carbonate of lime withstands the highest tem- 
peratures and is not to be decomposed by heat, but that when a little steam is 
thrown upon the heated carbonate, decomposition takes place with facility. Why is 
this? but that the water is the medium for the transference of the polar action which 
can now take place with the division of the masses : thus — 
+ — H — 
Ca CO3 H0=Ca0+C03 H. 
(s ) 
I might mention many other examples of the same class. 
On the view which I have here given of the nature of chemical change, the very 
existence of the elemental bodies was a strange and unaccountable anomaly. I have 
regarded the molecular structure of bodies but as an expression of the law of their 
synthesis and analysis, and this law again as a result of the peculiar nature of che- 
mical force. It was therefore truly difficult to conceive how an element in the sense 
of Berzelius was formed. The formation of this “ uncombined particle ” was a fact 
quite different to the formation of a compound substance, and yet it seemed as 
unreasonable to suppose that the laws of chemical action should vary in different 
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