TRANSACTIONS OF SECTION B. 667 



other substances are concerned, points now given expression to by such terms ag 

 ' chemical constitution,' ' homologous' and 'analogous series,' ' Kopp's laM',' &c. 



What is so important to consider in the study of double decomposition is that 

 the fact, that the sum of the masses of the two products of the change is the sum 

 of the masses of the two iuteractiug substances, presents itself no longer as being 

 merely the evidence of the massing together of substances into a compound ; for 

 there is in double decomposition to be considered that redistribution of mass 

 which, on the one hand, is found to correspond to and be part of a general 

 though not sharply defined redistribution of physical and chemical properties; 

 and, on the other hand, to be obviously irreducible to that interchange of those 

 simpler substances which in many cases are produced in the simple decomposition 

 of the acting substances. 



The physical properties of substances, or rather their sensible qualities, are of too 

 uncertain a character for their redistribution to be safely traced. But it generally 

 does result, amongst inorganic substances, at least, that colour is transmitted, the 

 saline, acid, bitter, or other taste of one of the active substances will appear, with 

 more or less distinctness, in one of the products, a relatively volatile and a relatively 

 fixed substance together will yield a similar pair of products, a dense and a light 

 substance will yield a dense and a light substance, and so on. The chemical 

 properties, however, are quite definitely redistributed to a large extent, a fact 

 sufficiently illustrated by saying that an iron salt yields an iron salt, and a sulphate 

 yields a sulphate. 



But this is not a redistributiou in which simpler substances, or indeed any 

 other substances than those iuteracting, play a part : as soon becomes evident 

 on attempting to establish the contrary by an appeal to the facts. While 

 silver acetate and silver sulphate resemble each other and also silver nitrate as 

 silver salts, they do not resemble silver itself; and though silver nitrate resembles 

 sodium nitrate as nitrate, there is not even a substance known which is related to 

 these salts as silver is related to silver salts. It might be objected to this that 

 there maj- yet become known such a substance, which in its ultimate decomposi- 

 tion would give one molecule of nitrogen to three molecules of oxygen. If instead 

 of nitrate were given acetate or cyanide, there would be found in the substances 

 acetic peroxide and cyauogen, it might be said, the analogues of the as yet unknown 

 substance of the nitrate. But the point I would make is that nitrate, sulphate, 

 &c., are names with well-defined meanings independent of the fact that the cor- 

 responding substances are not known ; for it follows without argument that also 

 the terms silver, iron, chloride, &c., should be equally independent in meaning of 

 the existence of the substances silver, sodium, chlorine, &c. It is a familiar 

 historical fact that ceesium, helium, and fluorine were chemical names long before the 

 substances C83sium,helium, and fluorine became known. We might well be convinced, 

 therefore, without going further, that constitutional names, names which convey 

 the facts of likenesses preserved in chemical change, cannot be indicators of the 

 presence of the substances for which they may be also used. For, that being the 

 case, we have no grounds for assuming that silver nitrate in interaction with 

 sodium sulphate decomposes into the substance silver, which then combines to 

 form silver sulphate. But fuller proof than any appearance of likeness or unlike- 

 ness can give is afibrded by facts which became known and appreciated in 

 connection with the chemical molecule. Ty])ical of them all is the fact that 

 in none of its interactions does chlormethane yield a hydrocarbon simpler than 

 methane or than itself Under those conditions in which' it might have been ex- 

 pected to give a substance which would be methyl, it produces ethane, a substance 

 which chlorine converts into another substance, having instead of one- third only 

 one-sixth less hydrogen in its composition. Similar results have been obtained in 

 all cases where the point can be determined — that is, where the simpler substance 

 looked for would still be a compound substance, and such simpler derivatives are 

 looked for no longer. The monohydride of oxygen or sulphur, the dihydride of 

 nitrogen or phosphorus or arsenic, the mononitride of carbon, the organic com- 

 pounds, methyl, phenyl, acetyl, are not only unknown, but are held to be uon- 

 existejit substances, though their chemical compounds, the hydroxides, amides, 



