198 
logous series in organic chemistry, afford further evidence 
in support of the theory that elementary species are formed 
by the successive condensations of a primordial substance of 
small specific gravity and low atomic weight. The physical 
and chemical properties of hydrogen, especially its low 
atomicity and its exact multiple relations with many ele- 
mentary substances, long since suggested to Prout that this 
element might be the ponderable base of all the others.* 
P rout’s hypothesis has not, however, made much progress, 
as chemical knowledge was not sufficiently advanced in his 
time to enable the intermediate steps to be perceived by 
which elements of high atomicity could be built up from 
hydrogen; and, besides this, the hypothesis afforded no 
explanation of the widely diverging properties of elements 
having nearly the same atomic weights. If, however, it be 
assumed that a particle of hydrogen combines successively 
with one, two, three, or more of its own particles, to form 
the molecules H2, H3, H4, H5, H 6, H7, and that each of 
these molecules forms the type of a group of elements under 
it, the intermediate steps between the low atomic weight of 
hydrogen, and the high atomic weights of other elements 
are perceived, and the different properties of elements of 
approximately equal atomic weights admit of a rational 
explanation. 
Although it is herein assumed that hydrogen is the 
ponderable base of all elementary species, it is probable 
that this element itself, as further maintained by Prout, 
may have been evolved from an ethereal substance of much 
greater tenuity.*)' Further knowledge of the outer regions of 
the solar atmosphere, and of the zodiacal light, may possibly 
indicate the steps by which hydrogen was formed. 
I would also observe that the term “molecule” is here 
used only in the sense of a larger or denser particle of 
* Annals of Philosophy, vol. vi., 330, 1815 ; vol. vii., 113, 1816. 
t Prout’ s Chemistry and Meteorology, 8th Bridgewater Treatise, p. 130. 
