ON THE LAW OF AVOGADRO AND AMPERE.
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 C2=24; that of nitrogen N2=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 C2 and N2; but there is this essential difference, that the specific gravity of cyanogen gas, 26, coincides with the molecular weight, while the assumed molecular weight, N2=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.
5. Is the molecular weight not in every instance = two volumes?
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 H3N to two vols. of NH4Cl 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.
E. VOGEL.