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SELECTED ARTICLES. 
vapor. How can the thickening he hut by the formation of 
the groups ? Without this be admitted, the fact is unaccount- 
able. And on this ground calulation gives 6 ¥ ° for the specific 
gravity of sulphur vapor. This also gives a reason for the 
fact, that sulphur crystallizes in two incompatible forms: 
Calculation gives for the simple vapors of arsenic and phos- 
phorus, the specific gravities 5i and 2§; but the doubles of 
these, viz. 10^ and 4-J, are very nearly the experimental re- 
sults, showing that their vapors are in single groups of two 
atoms each; from this arises their remarkable isomorphism. 
Hence this rule for finding the specific gravity of all gaseous 
bodies, simple or compound, — viz., add together the atomic 
weights of the atoms composing a single group, and multiply 
this sum by -IA 9 the result is the specific gravity required. 
When the gas is in double groups, half the sum of the 
weights of its atoms is the weight of a single group; and 
atoms united by cohesive combination may be treated as 
mixed gases. Thus we may be sure that the following num- 
bers are the specific gravities of the substance examined, viz.- — 
V°> T§> V* TT' and V> or in decimals, N = .9722 
= 1.1111, H = > 0694, Cl = 2.5, C = .8333 S*= 2.2222, As= 
5.2777, and P==2.2222. The facts above stated show that 
no number nearer the truth than these can be fixed on; hence 
they become eight important constants, which, if received as 
such, will spread their influence through chemistry, and give 
to it an uniformity very desirable in chemical research and 
nomenclature: it will make the language of chemistry more 
universal and easy, on which account it would yield me plea- 
sure if I could induce expert and experienced chemists, who 
are furnished with suitable apparatus, to experiment carefully 
on these substances, particularly on nitrogen gas, in order to 
confirm these results, or to correct them if requisite. At the 
commencement of this paper, I asserted that no physical 
theory could be true, unless it admit a change of direction in 
the acting forces. This I demonstrated in respect of 
Mossotti's theory, (in the Phil. Mag, December, 1S37,) and 
also in respect of Mr. Babbage's modification of it, where 
two sorts of atoms are supposed, each repelling its own 
