[ 180 ] 



XXI. Chemical Symmetry, or the Influence of Atomic Arrange- 

 ment on the Physical Properties of Compounds. By Thomas 

 Carnelley, D.Sc, Professor of Chemistry in Firth College, 

 Sheffield. 



[Concluded from p. 130.] 



II. Influence of Atomic Arrangement on the Solubility. 

 \ ITE have seen that those compounds whose atoms are 

 ▼ ▼ symmetrically arranged melt higher than those in 

 which the arrangement is asymmetrical. Now let us see if 

 there are not other physical properties which vary in this 

 respect in the same way as the melting-point. 



All bodies are acted on by at least two forces; one of which 

 is the attraction between the molecules, and the other (viz. 

 heat) tends to drive them apart. When the first force is in 

 excess of the second, the body is solid ; when the second is in 

 excess, the body is a gas ; when the two are about equally 

 balanced, the body is liquid. Therefore the greater the attrac- 

 tion between the molecules the greater the temperature re- 

 quired to melt the body, and consequently the higher the 

 melting-point. Having come to this conclusion, let us see if 

 there are not other physical properties which depend on the 

 attraction between the molecules, and which hence ought to 

 be affected by symmetry of arrangement of the atoms in the 

 molecule in the same way as the melting-point. 



The first of such properties which suggests itself is solubility. 

 In order that a solid may dissolve in any liquid, it is necessary 

 that its molecules should undergo a sort of unloosening process; 

 and we should therefore conclude that, of two isomeric com- 

 pounds, that would dissolve the most easily in which the 

 atttraction between the molecules was the least, and which 

 would therefore, as shown above, have the lowest melting- 

 point. This argument shows, then, that of two isomeric 

 bodies that dissolves the most easily which has the loioer melting- 

 point, and in which therefore the atomic arrangement is less 

 symmetrical. 



Let us now apply this rule in a few cases, and see whether 

 symmetrical compounds are less soluble than their asymme- 

 trical isomers. The following instances were taken quite pro- 

 miscuously from Watts's ' Dictionary,' and are all that were 

 tried, whether agreeing or not with the rule. The numbers 

 in the third column represent the order of solubility; whilst 

 the melting-point is taken as a measure of the symmetry: — 



