513 
PHAEMACEUTICAL MEETING, EDINBUEGH. 
A meeting was held in St. George’s Hall on Thursday evening, 5th March ; 
Mr. Young, President, in the chair. 
The following paper was read on— 
CHEMICAL CONSTITUTION AND ITS EELATION TO PHYSIO¬ 
LOGICAL ACTION. 
BY DR. A. CRUM BROWN, F.R.C.P. EDIN., ETC. 
Chemical composition is an expression which has a perfectly definite meaning. 
By it we understand the proportions of the elements (or hitherto undecomposed 
substances) which can be obtained from a substance or from which it can be 
built up. 
In investigating the composition of a substance it is a matter of secondary 
importance how it has been decomposed or how it may be reconstructed ; what 
we wish to know is, what are its ultimate constituents, and how much of each 
it contains. The knowledge of the intermediate steps is only valuable so far as 
it is necessary to prove that we have lost nothing and added nothing. It is 
quite different when we come to consider chemical constitution. Here we have 
to deal not only with the nature and quantity of the constituent elements, but 
with their relations to each other in the compound. The difference between 
composition and constitution may be illustrated by very many analogies. I 
shall make use of a familiar one. The composition of a household is expressed 
by the number of persons contained in it, with the sex and age of each, but its 
constitution is not known, until we have the relations in which they stand to 
one another, as husband and wife, parents and children, master and servant, 
etc. 
If we assume the existence of atoms, we may define constitution as the rela¬ 
tion of the atoms to one another in the compound, just as the constitution of a 
family is the relation of the individuals composing it to one another. We have 
now to inquire, how these relations may be discovered, and how they may be 
expressed. We discover them by examining the way in which the compound is 
built up or decomposed, and we express them by means of “ rational ” or 
“ constitutional ” formulae. In the case of the family, alluded to above, we 
should know the constitution if we knew the history, that is, if we knew by 
what steps these particular individuals came to live together in one house ; and 
in the same way, the history of a compound, or the steps by which it was pro¬ 
duced from its elements, leads us to a knowledge of its constitution. As, 
however, the work of destruction is generally easier than that of construction, 
we are often obliged to take the course of gradually, so to speak, picking the 
compound to pieces, and learning its history backwards. 
It would be out of place for me here, even did time permit, to give detailed 
examples, showing how the constitution of particular substances may be deter¬ 
mined ; a very clear account of the subject will be found in the article, 
“Formulae, Eational,” by Professor G. C. Foster, in Watts’s ‘Dictionary of 
Chemistry.’ It is sufficient here to indicate some of the general results. We 
believe, then, that the atoms in a compound are related to one another in such 
a way that atom is united to atom, and that when two groups of atoms unite 
together, it is by individual atoms of the one uniting with individual atoms of 
the other. We find that the atoms of some elements can each enter into only 
one relation, those of some into two, those of some into three, etc.; such atoms 
are called monatomic, dyatomic, triatomic, etc., respectively ; if w T e prefer 
English to Greek names, we may call them singly-related, doubly-related, and 
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