These conclusions overthrow all the fundamental assumptions on which the
hypothesis rests, and leave it, in the full meaning of the term, without
support. Though Mr. Greene states that my arguments are based upon
entirely erroneous premises, he has not even attempted to invalidate a
single one of my premises.
As he considers the non-condensation to be natural in the case of
cyanogen and chlorine, the condensation of two vols. of HCl + two vols.
of H_3N to two vols. of NH_4Cl ought to appear to him unnatural. He,
however, contends for it, and tries, on this solitary occasion, to
strengthen his opinion by authority, though the proof, if it could be
given, that ammonium chloride at the temperature of volatilization is
decomposed into its two constituents, would be insufficient to uphold
the theory.
The ground on which Mr. Greene assumes a partial decomposition at 350°
C. is the slight excess of the observed density (14.43) over that
corresponding to four vols. (13.375). There is, however, a similar
slight excess in the case of the vapor of ammonium cyanide, the same
values being respectively 11.4 and 11; and as this compound is volatile
at 100° C and, at the same time, is capable to exist at a very high
temperature, being formed by the union of carbon with ammonia, nobody
has ever, as far as I am aware, maintained that it is completely or
partially decomposed at volatilization. The excess of weight not being
due, therefore, to such cause in this case, it cannot be due to it in
the other.
The question being whether the molecular weight of ammonium chloride
is two vols. or four vols., an idea of the magnitude of the assumed
decomposition is conveyed by the proportion of the volume of the
decomposed salt to the volume of the non-decomposed, and Mr. Greene's
quotation of the percentage of weight is irrelevant and misleading, and
his number not even correct. A mixture containing
1.055 vols. of spec. gr. 26.75 = 28.22 and
12.32 " " " " 13.375 = 164.78
------ ------
13.375 " 193
has the spec. gr. 193 / 13.375 = 14.43. The proportion in one vol. of
the undecomposed to the decomposed salt is, therefore, as 1 to 11.68 and
the percentage of volume of the former 0.0789, and that of weight 28.22
/ 193 = 0.146, and not 0.16.
It is not easy to imagine why a small fraction of the heavy molecules
should be volatilized undecomposed, the temperature being sufficient
to decompose the great bulk. Marignac assumes, indeed, partial
decomposition, but the difficulties which he encountered in making the
experiments, on the results of which his opinion rests, were so great
that he himself accords to the numbers obtained by him only the value of
a rough approximation.
The heat absorbed in volatilization will comprise the heat of
combination as well as of aggregation, if decomposition takes place, and
will therefore be the same as that set free at combination. Favre and
Silbermann found this to be 743.5 at ordinary temperature, from which
Marignac concludes that it would be 715 for the temperature 350°; he
found as the heat of volatilization 706, but considers the probable
exact value to be between 617 and 818.[1]
[Footnote 1: See _Comptes Rendus_, t. lxvii., p. 877.]
An uncertainty within so wide a range does not justify the confidence
of Mr. Greene which he expresses in these words: "It is, therefore,
extremely probable that ammonium chloride is almost entirely
dissociated, even at the temperature of volatilization." By Boettinger's
apparatus a decomposition may possibly have been demonstrated, but it
remains to be seen whether it is not due to some special cause.
When Mr. Greene says that the relations between the physical properties
of solids and liquids and their molecular composition can in no
manner affect the laws of gases, nobody is likely to dissent; but the
conclusion that their discussion is foreign to the question of the
number of molecules in unit of volume does by no means follow. If the
specific gravity of a solid or the weight of unit of volume represents
a certain number of molecules, and is found to occupy two volumes in a
compound of the solid with another solid, the number of molecules in one
volume is reduced to one half. This I have shown to be the case in a
number of compounds, and the decrease of the specific gravity with
increase of the complexity of composition appears to be a general law,
as may be concluded from the very low specific gravity of the most
highly organized compounds, for instance the fatty bodies, the molecules
of which, being composed of very many constituents, are of heavy weight;
and likewise the compounds which occur in combination with water and
without it, the simpler compound having invariably a greater specific
gravity than the one combined with water; for instance:
BaH_2O_2 sp. gr. 4.495
" " + 8H_2O " 1.656
S_2H_2O_2 " 3.625
" " + 8H_2O " 1.396
FeSO_4 " 3.138
" + 7H_2O " 1.857
and so in every other case. This is now a recurrence of what takes
place in gases, and proves the fallacy of the hypothesis; for if these
compounds could be volatilized the vapor densities would necessarily
vary in the inverse proportion of the degree of composition.
The reproach that Berthelot has been endeavoring for nearly a quarter of
a century to hold back the progress of scientific chemistry, is a great
and unjustifiable misrepresentation of the distinguished chemist
and member of the Institute of France, who has done so much for
thermo-chemistry, and the more unfortunate as it seems to serve only the
purpose of a prelude to the following sentences: "But Mr. Vogel cannot
claim, as can Mr. Berthelot, any real work or experiment, however
roughly performed, suggested by the desire to prove the truth of his
own views. Let him not, then, bring forth old and long since explained
discrepancies, ... but when he will have discovered new or overlooked
facts ... chemists will gladly listen." ... Mr. Greene is here no longer
occupied to investigate whether what I have said concerning Avogadro's
hypothesis is true or false, but with myself he has become personal, and
in noticing his remarks my sole object is to contend against an error
which is much prevalent. If, according to Mr. Greene, the real work of
science consists in experimenting, and conclusions unsupported by our
own experiments have no value, it does not appear for what purpose he
has published his answer to my paper; an experiment of his, settling
Marignac's uncertain results, would have justified the reliance he
places on them. The ground he takes is utterly untenable. Experiments
are necessary to establish facts; without them there could be no
science, and the highest credit is due to those who perform successfully
difficult or costly experiments. Experimenting is, however, not the
aim and object of science, but the means to arrive at the truth; and
discoveries derived from accumulated and generally accepted facts are
not the less valuable on account of not having been derived from new and
special experiment.
It is, further, far from true that the real work of science consists
in experimenting; mental work is not less required, and the greatest
results have not been obtained by experimenters, but by the mental labor
of those who have, from the study of established facts, arrived at
conclusions which the experimenters had failed to draw. This is
naturally so, because a great generalization must explain all the facts
involved, and can be derived only from their study; but the attention
of the experimenter is necessarily absorbed by the special work he
undertakes. I refer to the three greatest events in science: the
discovery of the Copernican system, the three laws of Kepler, and
Newton's law of gravitation, none of which is due to direct and special
experimentation. Copernicus was an astronomer, but the discovery of his
system is due chiefly to his study of the complications of the Ptolemaic
system. Kepler is a memorable witness of what can be accomplished by
skillful and persistent mental labor. "His discoveries were secrets
extorted from nature by the most profound and laborious research." The
discovery of his third law is said to have occupied him seventeen years.
Newton's great discovery is likewise the result of mental labor; he was
enabled to accomplish it by means of the laws of Kepler, the laws of
falling bodies established by Galileo, and Picard's exact measurement of
a degree of a meridian.
If, then, mental work is as indispensable as experimental, it is not
less true that there are men more specially fitted for the one, others
for the other, and the best interests of science will be served when
experiments are made by those specially adapted, skillful, and favorably
situated, and the possibly greatest number of men, able and willing to
do mental work, engage in extracting from the accumulated treasures of
experimental science all the results which they are capable to yield.
Any truth discovered by this means is clear gain, and saves the waste
of time, labor, and money spent in unnecessary experiment. Mr. Greene's
zeal for experiment and depreciation of mental work would be in order,
if ways and means were to be found to render the advancement of science
as difficult and slow as possible; they are decidedly not in the
interest of science, and can not have been inspired by a desire for its
promotion.
As the evidence of the specific heats of the fallacy of Avogadro's
hypothesis involves lengthy explanations, the subject is reserved for
another paper.
San Francisco, Cal., May, 1881.