ADDllESS. IxXV 



compound undistinguishable from the first, so that, in an aggregate of like 

 molecules, the apparent atomic rest is the result of the interchange of like 

 atoms between contiguous molecules. 



Such exchanges of atoms take place not only between molecules of iden- 

 tical composition, but also between contiguous molecules containing different 

 elements. For instance, in a mixture of sodic chloride and potassic iodide 

 an interchange of metallic atoms takes place, forming potassic chloride and 

 sodic iodide. The result of the exchange in such a case is to form a couple 

 of new molecules diff^erent from the original couple. But these products are 

 subject to the same general law of atomic exchanges, and their action on one 

 another reproduces a couple of molecules of the materials. 



Thus a liquid mixture formed from two compounds, contains molecules of 

 four kinds, which we may describe as the two materials and the two products*. 

 The materials are reacting ou one another, forming the products ; and these 

 products are, in their turn, reacting on one another, reproducing the materials. 



If one of the products of atomic exchange between two molecules is a solid 

 while the otlier remains liquid (as when sodic chloride is mixed with silver 

 nitrate), or if one is gaseous while the other remains liquid, so that the 

 molecules of the one kind cannot react on those of the other kind and re- 

 produce the materials, then the continued reaction of the materials on one 

 another leads to their complete mutual decomposition. Such complete mu- 

 tual decomposition of two salts takes place whenever they react on one 

 another under such conditions that the products cannot react on one another 

 and reproduce the materials; whereas partial decomposition takes place 

 whenever the materials form a homogeneous mixture with the products. 



Now, if in any such homogeneous mixture more exchanges of atoms take 

 place between the materials than between the products, the number of mole- 

 cules of the products is increased, because more of them are being made than 

 unmade ; and reciprocally, if more exchanges of atoms take place between 

 the products than between the materials, the number of molecules of the 

 materials is increased. The mixture remains of constant composition when 

 there are in the unit of time as many decomposing changes as reproducing 

 changes. 



Suppose that we were to determine by experiment the proportion between 

 the number of molecules of the materials, and the number of molecules of 

 the products, in a mixture the composition of which remains constant, and 

 that we found, for instance, twice as many of materials as of products ; what 

 would this mean ? Why, if every two couples of materials only effiect in the 

 unit of time as many exchanges as every one couple of products, every couple 

 of materials is only exchanging half as fast as every couple of products. 



In fact you perceive that a determination of the proportion in which the 

 substances are present in such a mixture will give us a measure of the rela- 

 tive velocities of those particular atomic motions ; and we may thus express 

 our result : — The force of chemical combination is inversely proportional to 

 the number of atomic interchanges. 



I cannot quit this part of our subject without alluding to the fact that 

 some few chemists of such eminence as to be entitled to the most respectful 

 attention, have of late years expressed an opinion that the idea of atoms is 

 not necessary for the explanation of the changes in the chemical constitution 

 of matter, and have sought as far as possible to exclude from their language 

 an}r allusion to atoms. 



It would be out of place on this occasion to enter into any discussion of 

 the questions thus raised ; but I think it right to point out : — 



