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XXXVII, On the Condition of certain Elements at the moment of Chemical Change. 
By Benjamin Collins Brodie, Esq., F.R.S. 
Received June 6, — Read June 20, 1850. 
The experimental inquiry which I now lay before the Society is so intimately con- 
nected with certain theoretical considerations, in which it took its rise, and which 
are necessary to its right comprehension, that I am unwilling to separate them. 
These considerations alone can explain why it appeared to me desirable to devote 
much time and labour to the determination of a simple analytical problem, which it 
has been open for the last thirty years to any chemist to undei'take, but which, 
although doubtless connected with some of the most curious and obscure phenomena 
of chemical science, no one has thought it worth his while to enter upon. The reason 
of this may have been, that from other points of view this inquiry seemed of little 
importance, or, which is also probable, the question may have been, at various times, 
partially investigated, and the answer to it thought to be other than what it truly is, 
because approached from a wrong side. I shall therefore lay before the Society 
theory as well as experiment ; for the theory is necessary to render the experiment 
intelligible, although this latter, in so far as it is true, has an independent value, and 
may be explained by others in some totally different manner. 
The difference which chemists draw between the chemical elements and all other 
bodies, is far greater and indeed of altogether another kind to that which exists be- 
tween any two compound substances. Other bodies are composed and decomposed ; 
but applied to the elements, these words are altogether inappropriate; when the ele- 
ment is formed there is no chemical synthesis, and when it passes from the free to 
the combined state, there is no chemical decomposition. Tliis difference the atomic 
theory expresses by assigning to the two classes of bodies a different molecular con- 
stitution. The element it considers as consisting of single and isolated atoms, and 
all other bodies as systems more or less complex of combined particles. This fun- 
damental difference of conception is yi^ell given in the following passage from Ber- 
zelius*: — “Les atomes d’un meme corps elementaire ne possMent aucune force de 
combinaison mutuelle ; ils n’adherent ensemble qu’en vertu de la force d’agregation. 
Plus deux corps Hementaires se ressemblent quant a leurs proprietes chiiniques, inoins 
ils tendent a s’unir, et le nouveau corps qui resulte de leur combinaison, a tant d’ana- 
logies avec ses elements constituants, qu’il diff^re pen d’un mHange mecanique. 
Plus, au contraire, les corps elementaires different de proprietes chiiniques, plus la 
* Berzelius, Traite de Chimie, Paris, 1845, vol. i. p. 25. 
