54 The Discovery of the Atomic Theory. 
lumn of quicksilver, under the pressure of which aqueous vapour 
would begin to condense at the temperature required for expelling 
the water out of these earthy hydrates: and this height would 
express'the mutual ailinity of the alkaline earths and water, and 
might be compared at pleasure with any other mechanical power. 
And since chemical attractions may be compared with each 
other, we might perhaps by degrees advance so far by these 
means, as to be able to express every chemical affinity in numbers, 
and to compare it with gravity, the universal measure of me- 
chanical force. 
We shall hereafter see that some of the weaker bases, when 
they take up water, retain it with a force so moderate, as to be 
exceeded by the expansive power of the water even at lower 
temperatures, Some of them thus become in some measure 
hygroscopic substances, since their ereaten or less approach to 
the maximum of moisture depends ou the dryness of the air, and 
only takes place w hen the air exhibits a similar maximum, and 
the expansive power of the water at the given temperature is 
completely annihilated. 
j [To be continued. ] 
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XI. The Discovery of ithe Atomic Theory claimed for 
Mr.Hieains, By Joun Nasu, Esq. 
, Merrion Square, Dublin, Jan. 7, 1814. 
Sir, I BEG leave to call the attention of your readers to a pub- 
lication which has recently appeared in a SOVseinae ary Journal, 
and to point out a mis-statement there given to the public, in- 
jurious, in my opinion, to the scientific reputation of a learned 
and respected individual. 
The passage I allude to is contained in No. 12 of Dr. Thom- 
son’s Annals of Philosophy, and is in these words : 
<< When we reflect on this cause, it appears at first evident that 
it must be of a mechanical nature; and what presents itself as 
the most probable idea, mest conformable to our experience, is, 
that bodies are composed of atoms, or of molecules which com- 
bine 1 with 1, | with 2, or 3, 4, &c.; and the laws of chemical 
proportions seem to result from this with such clearness and 
evidence, that it seems very singular that an idea so simple and 
probable has not only not been adopted, but not even proposed 
before our own days. As far as I know, the English philosopher 
Mr. John Dalton, guided by the experiments of Bergman, Richter, 
Wenszel, Berthollet, Proust, and others, was the first person who 
endeavoured to establish that hypothesis. Sir Humphry Davy 
has lately assured us, that Mr. Higgins in a book published in 
the year 1789 established the same hypothesis, 1 have not. seen 
the 
