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Proceedings of the Royal Society 
variable volume-equivalents, furnishes the basis of calculation. 
Accordingly, our tree of chemical knowledge splits up into two 
main branches, to which we might apply the terms Pondo-che- 
mistry and Impondo-chemistry. I beg to remark, in conclusion, 
that having resolved upon a minute and thorough investigation 
of this, no doubt very intricate, subject, I was surprised to find, 
after examining all the principal sources of information, that a 
complete and trust-worthy catalogue of the leading physical pro- 
perties of matter was still a desideratum, but more particularly 
that the determination of the specific gravities of substances in 
the liquid and solid state had been much neglected. With the 
most ample resources at my disposal, I have at length succeeded 
in composing a catalogue of these latter. The number of hona 
fide cases entered therein amounts to about 1800 ; but this number 
is after all a mere fraction, in comparison with the hundreds of 
thousands of non-determined bodies. I may also state, that should 
a systematic revision of this most needful portion of our science 
be resolved upon, I shall feel great pleasure in presenting my 
catalogue, such as it is, to the parties entrusted with its execu- 
tion. 
6. On the Theory of Isomeric Compounds. By Dr A. Crum 
Brown. 
In this paper only those bodies are considered which are abso- 
lutely isomeric,” that is, which have not only the same composition 
per cent, and the same molecular weight, but also the same constitu- 
tional formula. As the constitutional formula of comparatively 
few substances is known, this class is as yet a small one. 
The following pairs of substances are probably absolutely iso- 
meric. 
1. The alcohol radicals and the hydrides, as methyl gas and the 
hydride of ethyl. 
2. Chloride of ethyl and the product of the action of chlorine on 
hydride of ethyl. 
3. Chloride of vinyl and chloracetene. 
4. Fumaric and maleic acids. 
