TRANSACTIONS OF SECTION B. 571 



the word. I plead for a return to the endinf^ of the word radical with ' al,' now 

 interdicted in the ' Journal of the Chemical Society.' It seems appropriate to call 

 the powers of a substance to behave chemically as it does, the roots or radical 

 parts of its chemical nature, but it does not seem appropriate to call them radicles 

 or rootlets. Americans and all other nationalities but our own use the criffinal 

 spelling. 



I have put off too long, perhaps, all refereuce to the properties of very dilute 

 aqueous solutions of salts, but I wished first to discuss the nature of tlie radical. 

 The osmotic pressure and other dependent points which are particular in the 

 behaviour of such solutions are in full accordance with the assumption that an 

 electrolyte by dissolution in much water becomes a pair or a binary system of 

 two interdifl'used quasi-substances called ' ions.' These ions must differ from 

 isolated substances in bearing equal and opposite quantities of electricity ; in 

 being each unknown apart from its fellow ; and in having a composition not 

 to be found in actual substances, though identical possibly with that which 

 a radical would have were it a substance. The ions can be indeed separated 

 from each other, but not to continue as themselves, since in the act of 

 separating they form ordinary substances, either by uniting with other ions, 

 or by two molecules of ion becoming one molecule of substance. In the 

 former way of separation the ions of two salts interact on mixing their 

 solutions ; in the other way, the ions become substances when their solution is 

 placed in a galvanic circuit. In this mode of separation — by electrolysis, that 

 is — the substances corresponding with the two ions, or else secondary products of 

 their change, are produced, the one substance at the kathode and the other at the 

 anode, while the solution away from the electrodes, but between them, remains for 

 the time unaltered in composition. Along with this there occurs in many cases 

 a phenomenon first recorded by Daniell, and afterwards investigated by Hittorf 

 with such beautiful results. This consists in a greater fall taking place in the 

 concentration of the salt solution close to one electrode than in the concentration 

 of that close to the other, as though the ions were hydrate compounds, and that 

 the one ion was a higher hydrate than the other. Until we know more of the 

 nature of the ions themselves this phenomenon is most conveniently quantified 

 on the hypothesis that the ions travel as molecular particles, but the discussion 

 of this hypothesis is beside my present purpose. 



The phenomena of ionisation or, in other words, the particular properties of 

 dilute solutions of salts, belong evidently to a change unlike all other chemical 

 changes. It is a polarised chemical change, in which the equivalent and comple- 

 mental products of the interaction appear apart and at remote surfaces of the mass 

 of decomposing salt solution. Two points which call for notice in connection with 

 my present subject are that an ion is one of a pair of quantities commensurate with 

 the quantity of the salt itself that is or would be in interaction ; and that it is 

 molecular in character and therefore to be regarded as a relative and wholly 

 variable quantity. 



Dalton's atoms were both the atoms and the molecules of present-day 

 chemistry, but much more the latter than the former. Although the chemical 

 atom can now be no more than a dependency of the molecule, it is commonly set 

 up as the starting-point in chemical theory, and as having an independent existence 

 as a quantity of the substance, while the molecule is represented as being a con- 

 jugation of atoms. But there cannot be two standards in reference to the same 

 thing, and in molecular chemistry the atom must give way. As I have already 

 had occasion to point out, the atom is of the radical, the molecule is of the 

 substance. 



The four radicals of a double decomposition are equal and chemically com- 

 plementary. These chemically equal quantities of such radicals are atoms. The 

 quantities of all other radicals are also atoms, but only those of proximate radicals, 

 those of a single interaction, are equal. Similarly, the quantities of the four 

 substances of a single interaction are all equal and are molecules, but the quantities 

 of substances are not equal in other interactions. These others are treated as the 

 simultaneous occurrence of two or more single interactions, which they can always 



