The Scientific American Supplement of May 14,1881, contains, under this
head, Mr. Wm. H. Greene's objections to my demonstration (in No. 270
of the same paper) of the error of Avogadro's hypothesis. The most
important part of my argument is based on the evidence afforded by the
compound cyanogen; and Mr. Greene, directing his attention to this
subject in the first place, states that because cyanogen combines
with hydrogen or with chlorine, without diminution of volumes, I have
concluded that the hypothesis falls to the ground. This statement has
impressed me with the conviction that Mr. Greene has failed to perceive
the difficulty which is at the bottom of the question, and I will,
therefore, present the subject more fully and comprehensively.
The molecule of any elementary body is, on the ground of the hypothesis,
assumed to be a compound of two atoms, and the molecule of carbon
consequently C_2=24; that of nitrogen N_2=28. Combination of the two,
according to the same hypothesis, takes place by substitution; the
atoms are supposed to be set free and to exchange places, forming a
new compound different from the original only in this: that each new
particle contains an atom of each of the two different substances, while
each original particle consists of two identical atoms. The product is,
therefore, assumed to be, and can, under the circumstances, be no other
than particles of the composition CN and weight 26. These particles are
molecules, according to the definition laid down, just as C_2 and N_2;
but there is this essential difference, that the specific gravity of
cyanogen gas, 26, coincides with the molecular weight, while the assumed
molecular weight, N_2=28, is twice as great as the specific gravity of
the gas, N=14.
In using the term molecular weight, it is to be remembered that it does
not express the weight of single molecules, but only their relative
weight, millions of millions molecules being contained in the unit of
volume. But on the hypothesis that there is the same number of molecules
in the same volume of any gas, the specific gravities of gases can be,
and are, identified with their molecular weights, and, on the ground of
the hypothesis again, the unit of the numbers which enter into every
chemical reaction and constitute the molecular weight, is stipulated to
be that contained in two volumes.
The impossibility of the correctness of the hypothesis is now revealed
by the fact just demonstrated, that in the case of nitrogen the specific
gravity does not coincide with the molecular weight. If equal volumes
contain the same number of molecules, the specific gravities and the
molecular weights must be the same; and if the specific gravities and
molecular weights are not the same, equal volumes cannot contain the
same number of molecules. The assumed molecular weight of nitrogen is
twice as great as the specific gravity, but the molecular weight and
the specific gravity of cyanogen are identical; the number of molecules
contained in one volume of cyanogen must, therefore, necessarily be
twice as great as the number contained in one of nitrogen, and this is
fully and completely borne out by the chemical facts.
In saying that when cyanogen combines with chlorine there is naturally
no condensation, Mr. Greene has no idea that this natural law is fatal
to his artificial law of Avogadro and Ampere; "for," continues he, "the
theory is fulfilled by the actual reaction." It is not. The theory
requires two vols. of cyanogen and two vols. of chlorine, that is, the
unit of numbers, to enter into reaction and to produce two vols. of
the compound. But they produce four vols., and the non-condensation is
therefore in opposition to the theory. It is true beyond doubt that the
molecular weight of cyanogen chloride is contained in two volumes, in
spite of the hypothesis, not on the ground of it; two vols. + two vols.,
producing four vols.; two vols. could, theoretically, contain only half
the unit of numbers, and there seems to be no escape from the following
general conclusions:
1. Two vols. of CNCl, representing the unit of numbers, the constituent
weights, C=12, N=14, Cl=35.5, must each, likewise, represent the same
number; the molecular weight is, therefore, contained in one vol. of N
or Cl, but in two of CNCl and equal numbers are not contained in equal
volumes.
2. The weights N=14, Cl=35.5 occupy in the free state one volume, but
in the combination, CNCl, two volumes; their specific gravity is,
therefore, by chemical action reduced to one half. The fact thus
elicited of the variability and variation of the specific gravity is of
fundamental importance and involves the irrelevancy of the mathematical
demonstration of the hypothesis. In this demonstration the specific
gravity is assumed to be constant, and this assumption not holding good,
and the number of molecules in unit of volume being reduced to one half
when the specific gravity is reduced to the same extent by chemical
action, it is obvious that the mathematical proof must fail. Mr. Greene
states that I have proceeded to demolish C. Clerk Maxwell's conclusion
from mathematical reasoning. This is incorrect; I have found no fault
with the conclusion of the celebrated mathematician, and consider his
reasoning unimpeachable. I am also of opinion that he is entitled to
great credit and respect for the prominent part he has taken in the
development of the kinetic theory, and further think that it was for
the chemists to produce the fact of the variability of the specific
gravities, which they would probably not have failed to do but for the
prevalence of Avogadro's hypothesis, which is virtually the assertion of
the constancy of the specific gravities.
3. The unit of numbers being represented by Cl=35.5, it is likewise
represented by H=1, and as the product of the union of the two elements
is HCl, 36.5 = two vols., combination takes place by addition and not by
substitution; consequently are
4. The elementary molecules not compounds of atoms? And the distinction
between atoms and molecules is an artificial one, not justified by the
natural facts.