TRAN'SACTIOXS OF SECTIOX R. ."305 



with that of the atom, as the unit of measure of chemical activity in place of the 

 gravimetric unit ; the work of the chemists of the last mid-century gave us a 

 fuller conception of the molecule, along with the notion of chemical change as 

 being substitution in the molecule eflected by what became known as double 

 decomposition. Up to that time chemistry had been treated only as the science 

 of compounding and decompounding or reducing. Sodium added to oxygen gives 

 soda, sulphur added to oxygen gives sulphuric anhydride, soda added to the 

 anhydride gives sodium sulphate, ethylene added to chlorine gives dichlorethane, 

 water subtracted from alcohol leaves ether, and so forth. All this is strictly true 

 in a limited way, but then it is not chemistry ; and the addition precedes and 

 does not constitute the chemical union. In the sodium sulphate we perceive no 

 soda, no anhydride, no sodium, suljihur, or oxygen. That is to say, there is 

 evidence of the addition and subtraction of mass and some other such evidence; 

 but, for the rest, evidence of addition there is none. Were it otherwise there 

 could be no chemistry. It is true that one of the great things accomplished by 

 chemistry has been that of establishing the law of the conservation of mass, with- 

 out which to rely upon the chemist would be unable to carry on his experimental 

 investigations. But that is only because, like the steady point to the seismologist, it 

 is there unchangeable when all else is changing. Since it is the law of no change, 

 it cannot serve to explain what is change. Far from being the science of the compo- 

 .sition of substances, chemistry might be defined as being the science of the non- 

 composition of substances where that composition might have been looked for 

 from the antecedents. If salt is verily a compound of sodium and chlorine, and 

 can be broken up into these, why have the fragments not the marks on them of 

 that whole of which they formed a part ? It is true that 5850 parts of salt 

 become 3545 parts of chlorine and 2305 parts of sodium, nothing being gained 

 or lost in weight ; but to account for that there is no need of chemistry, a science 

 which takes cognisance of the phenomena of change, and not of those of un- 

 changed properties. The use of the word ' composition ' in chemistry cannot be 

 discarded now, and all that is necessary to make it unobjectionable is to see that 

 the term is always qualified by the prefix ' chemical ' when there is a possibility 

 of mistake about its significance, and that that significance is carefully explained, 

 if not defined and fully illustrated, before it is given over to the beginner. 



The facts of a chemical nature about common salt which cause the statement 

 to be made that it is a chemical compound of chlorine and sodium are such as these. 

 Salt can be wholly changed into sodium and chlorine ; these substances brought 

 together change into salt and nothing else ; salt and sodium, each under conditions 

 appropriate to it, change into the same substance, called also a sodium compound, 

 such as sodium hydroxide ; salt and chlorine, each in its own way, change into 

 the same chlorine compound, such as hydrochloric acid ; neither sodium nor 

 chlorine, one apart irom the other or the other's chemical compounds, ever changes 

 into salt ; salt is, directly or indirectly, producible in the chemical interaction of 

 a sodium compound with a chlorine compound ; the properties of salt are much 

 less like those of either sodium or chlorine than like those of some other sub-- 

 stances ; in sensible and other physical properties the chemically compound 

 substance, salt, is as simple as or simpler than either of the chemically simple 

 substances, sodium and chlorine ; lastly, the laws of combining proportion by 

 weight are obeyed in all the chemical changes in which salt takes part. 



AVith exclusive reference to such facts as these, the chemical composition of a 

 substance will, I think, be found to be satisfactorily defined, as its having the 

 power, capacity, or property of being wholly producible from and wholly con- 

 vertible into, directly or indirectly, those substances of which it is said to be 

 composed. A simple substance differs from one that is compound only in not 

 possessing the power of being by itself convertible into two others, or of being 

 produced alone from any two others. Simple substances are not less varied or 

 less complex in their physical properties than compound substances, while their 

 ohemical constitution is often more problematic than that of many which are com- 

 pound. The term ' simple,' therefore, is as misleading in the language of chemistry 

 as 'compound,' unless defined and qualified in use by the word ' chemically.' 



