﻿Royal Institution. 395 



ftOYAL INSTITUTION OF GREAT BRITAIN. 



March 19, 1869. — " On Chemical Constitution, and its Relation 

 to Physical and Physiological Properties." By Dr. A. Crum Brown, 

 F.R.S.E. 



Chemists have long endeavoured to answer the question, What 

 is the relation in which the constituents stand to one another in a 

 compound ? and numerous hypotheses, more or less ingenious, have 

 been devised for this purpose. Two of these modes of representing 

 chemical phenomena occupy so prominent a place in the history of 

 the science as to merit special notice, even in so slight and hurried 

 a sketch as this must be. These are, 1st, the Electro-chemical and 

 Radical Theory ; and 2nd, the Theory of Atomicity and Chemical 

 Structure. 



The first was the product of the genius, learning, and laborious 

 research of Berzelius ; it was soon adopted by all chemists, and 

 formed for many years the foundation of all chemical teaching and 

 the guide in all chemical work. The point of view from which it 

 regards chemical phenomena is that of combination and decomposi- 

 tion, of the union of elements to form compounds, and the separa- 

 tion of compounds into elements. A very important form of 

 chemical decomposition is electrolysis, or the breaking up of a com- 

 pound by means of current electricity. From the nature of the case 

 electrolysis gives rise to a dichotomous decomposition ; and this 

 duality was extended to all cases of combination and decomposition. 

 Elements combine with each other in pairs ; these pairs may again 

 combine in pairs, forming compounds of the second order, and so on. 

 Thus calcium combines with oxygen to form lime, sulphur combines 

 with oxygen to form sulphuric acid, and sulphuric acid combines 

 with lime to form sulphate of lime. This union of compounds with 

 compounds was not supposed to depend on a union of the con- 

 stituents of the one with the constituents of the other, but to be a 

 combination of the one as a whole with the other as a whole — not 

 a combination of the calcium of the lime with the sulphur or with 

 the oxygen of the sulphuric acid, or of the sulphur of the sulphuric 

 acid with the oxygen of the lime, but of the lime as such with the 

 sulphuric acid as such. 



This view may be illustrated by a reference to the relations of 

 human life. Individuals unite to form partnerships or corporations ; 

 and these may again enter into alliances, although the members of 

 the one allied corporation may be altogether unacquainted and 

 unconnected with the members of the other. 



But the progress of discovery brought to light facts which seemed 

 to contradict this view of binary 'combination. Cases were observed 

 in which a compound of two elements united directly with an 

 element ; and to meet this new class of facts the theory was modified 

 by the introduction of the notion of Radicals. A radical was a 

 compound which acts like an element. 



The simile introduced above may be used to illustrate this exten- 

 sion of the theory. Some combinations of men (corporations) can 

 be treated as individuals, can enter into legal relations with indivi- 



