Chapter VII., Note 26) and was found to be nearly 1·66, that is greater

than for those gases whose molecules contain two atoms (for instance, CO, H_{2}, N_{2}, air, &c., for which _k_ is nearly 1·4) or those whose molecules contain three atoms (for instance, CO_{2}, N_{2}O, &c., for which _k_ is about 1·3), but closely approximate to the ratio of the specific heats of mercury vapour (Kundt and Warburg, _k_ = 1·67). And as the molecule of mercury vapour contains one atom, so it may be said that argon is a simple gaseous body whose molecule contains one atom.[10] A compound body should give a smaller ratio. The experiments upon the liquefaction of argon, which we shall presently describe, speak against the supposition that argon is a mixture of two gases. The importance of the results in question makes one wish that the determinations of the ratio of the specific heats (and other physical properties) might be confirmed with all possible accuracy.[11] If we admit, as we are obliged to do for the present, that argon is a new element, its density shows that its atomic weight must be nearly 40, that is, near to that of K = 39 and Ca = 40, which does not correspond to the existing data respecting the periodicity of the properties of the elements in dependence upon their atomic weights, for there is no reason on the basis of existing data for admitting any intermediate elements between Cl = 35·5 and K = 39, and all the positions above potassium in the periodic system are occupied. This renders it very desirable that the velocity of sound in argon should be re-determined.[12]

[10] This portion of Rayleigh and Ramsay's researches deserves particular attention as, so far, no gaseous substance is known whose molecule contains but one atom. Were it not for the above determinations, it might be thought that argon, having a density 20, has a complex molecule, and may be a compound or polymerised body, for instance, N_{3} or NX_{_n_}, or in general X_{_n_}; but as the matter stands, it can only be said that either (1) argon is a new, peculiar, and quite unusual elementary substance, since there is no reason for assuming it to contain two simple gases, or (2) the magnitude, _k_ (the ratio of the specific heats) does not only depend upon the number of atoms contained in the molecules, but also upon the store of internal energy (internal motion of the atoms in the molecule). Should the latter be admitted, it would follow that the molecules of very active gaseous elements would correspond to a smaller _k_ than those of other gases having an equal number of atoms in their molecule. Such a gas is chlorine, for which _k_ = 1·33 (Chapter XIV., Note 7). For gases having a small chemical energy, on the contrary, a larger magnitude would be expected for _k_. I think these questions might be partially settled by determining _k_ for ozone (O_{3}) and sulphur (S_{6}) (at about 500°). In other words, I would suggest, though only provisionally, that the magnitude, _k_ = 1·6, obtained for argon might prove to agree with the hypothesis that argon is N_{3}, formed from N_{2} with the evolution of heat or loss of energy. Here argon gives rise to questions of primary importance, and it is to be hoped that further research will throw some light upon them. In making these remarks, I only wish to clear the road for further progress in the study of argon, and of the questions depending on it. I may also remark that if argon is N_{3} formed with the evolution of heat, its conversion into nitrogen, N_{2}, and into nitride compounds (for instance, boron nitride or nitride of titanium) might only take place at a very high temperature.

[11] Without having the slightest reason for doubting the accuracy of Rayleigh and Ramsay's determinations, I think it necessary to say that as yet (February 1895) I am only acquainted with the short memoir of the above chemists in the 'Proceedings of the Royal Society,' which does not give any description of the methods employed and results obtained, while at the end (in the general conclusions) the authors themselves express some doubt as to the simple nature of argon. Moreover, it seems to me that (Note 10) there must be a dependence of _k_ upon the chemical energy. Besides which, it is not clear what density of the gas Rayleigh and Ramsay took in determining _k_. (If argon be N_{3}, its density would be near to 21.) Hence I permit myself to express some doubt as to whether the molecule of argon contains but one atom.

[12] If it should be found that _k_ for argon is less than 1·4, or that _k_ is dependent upon the chemical energy, it would be possible to admit that the molecule of argon contains not one, but several atoms--for instance, either N_{3} (then the density would be 21, which is near to the observed density) or X_{6}, if X stand for an element with an atomic weight near to 6·7. No elements are known between H = 1 and Li = 7, but perhaps they may exist. The hypothesis A = 40 does not admit argon into the periodic system. If the molecule of argon be taken as A_{2}--_i.e._ the atomic weight as A = 20--argon apparently finds a place in Group VIII., between F = 19 and Na = 23; but such a position could only be justified by the consideration that elements of small atomic weight belong to the category of typical elements which offer many peculiarities in their properties, as is seen on comparing N with the other elements of Group V., or O with those of Group VI. Apart from this there appears to me to be little probability, in the light of the periodic law, in the position of an inert substance like argon in Group VIII., between such active elements as fluorine and sodium, as the representatives of this group by their atomic weights and also by their properties show distinct transitions from the elements of the last groups of the uneven series to the elements of the first groups of the even series--for instance,

Group VI. VII. VIII. I. II. Cr Mn Fe, Co, Ni Cu Zn

While if we place argon in a similar manner,

VI. VII. VIII. I. II. O = 16 F = 19 A = 20 Na = 23 Mg = 24

although from a numerical point of view there is a similar sequence to the above, still from a chemical and physical point of view the result is quite different, as there is no such resemblance between the properties of O, F and Na, Mg, as between Cr, Mn, and Cu, Zn. I repeat that only the typical character of the elements with small atomic weights can justify the atomic weight A = 20, and the placing of argon in Group VIII. amongst the typical elements; then N, O, F, A are a series of gases.

It appears to me simpler to assume that argon contains N_{3}, especially as argon is present in nitrogen and accompanies it, and, as a matter of fact, none of the observed properties of argon are contradictory to this hypothesis.

These observations were written by me in the beginning of February 1895, and on the 29th of that month I received a letter, dated February 25, from Professor Ramsay informing me that 'the periodic classification entirely corresponds to its (argon's) atomic weight, and that it even gives a fresh proof of the periodic law,' judging from the researches of my English friends. But in what these researches consisted, and how the above agreement between the atomic weight of argon and the periodic system was arrived at, is not referred to in the letter, and we remain in expectation of a first publication of the work of Lord Rayleigh and Professor Ramsey. [For more complete information see papers read before the Royal Society, January 31, 1895, February 13, March 10, and May 21, 1896, and a paper published in the Chemical Society's Transactions, 1895, p. 684. For abstracts of these and other papers on argon and helium, and correspondence, see 'Nature,' 1895 and 1896.

4. Argon was liquefied by Professor Olszewsky, who is well known for his classical researches upon liquefied gases. These researches have an especial interest since they show that argon exhibits a perfect constancy in its properties in the liquid and critical states, which almost[13] disposes of the supposition that it contains a mixture of two or more unknown gases. As the first experiments showed, argon remains a gas under a pressure of 100 atmospheres and at a temperature of -90°; this indicated that its critical temperature was probably below this temperature, as was indeed found to be the case when the temperature was lowered to -128°·6[14] by means of liquid ethylene. At this temperature argon easily liquefies to a colourless liquid under 38 atmospheres. The meniscus begins to disappear at between -119°·8 and -121°·6, mean -121° at a pressure of 50·6 atmospheres. The vapour tension of liquid argon at -128°·6, is 38·0 atmospheres, at -187° it is one atmosphere, and at -189°·6 it solidifies to a colourless substance like ice. The specific gravity of liquid argon at about -187° is nearly 1·5, which is far above that of other liquefied gases of very low absolute boiling point.

The discovery of argon is one of the most remarkable chemical acquisitions of recent times, and we trust that Lord Rayleigh and Professor Ramsay, who made this wonderful discovery, will further elucidate the true nature of argon, as this should widen the fundamental principles of chemistry, to which the chemists of Great Britain have from early times made such valuable contributions. It would be premature now to give any definite opinions upon so new a subject. Only one thing can be said; argon is so inert that its rôle in nature cannot be considerable, notwithstanding its presence in the atmosphere. But as the atmosphere itself plays such a vast part in the life of the surface of the earth, every addition to our knowledge of its composition must directly or indirectly react upon the sum total of our knowledge of nature.

[13] There only remains the very remote possibility that argon consists of a mixture of two gases having very nearly the same properties.

[14] The following data, given by Olszewsky, supplement the data given in Chapter II., Note 29, upon liquefied gases.

(_tc_) (_pc_) _t_ _t__{1} _s_ N_{2} -146° 35 -194°·4 -214° 0·885 CO -139°·5 35·5 -190° -207 ? A -121° 50·6 -187° -189°·6 1·5 O_{2} -118°·8 50·8 -182°·7 ? 1·124 NO -93°·5 71·2 -153°·6 -167° ? CH_{4} -81°·8 54·9 -164° -158°·8 0·415

where _tc_ is the absolute (critical) boiling point, _pc_ the pressure (critical) in atmospheres corresponding to it, _t_ the boiling point (under a pressure of 760 mm.), _t_{1}_ the melting point, and _s_ the specific gravity in a liquid state at _t_.

The above shows that argon in its properties in a liquid state stands near to oxygen (as it also does in its solubility), but that all the temperatures relating to it (_tc_, _t_, and _t_{1}_) are higher than for nitrogen. This fully answers, not only to the higher density of argon, but also to the hypothesis that it contains N_{3}. And as the boiling point of argon differs from that of nitrogen and oxygen by less than 10°, and its amount is small, it is easy to understand how Dewar (1894), who tried to separate it from liquid air and nitrogen by fractional distillation, was unable to do so. The first and last portions were identical, and nitrogen from air showed no difference in its liquefaction from that obtained from its compounds, or from that which had been passed through a tube containing incandescent magnesium. Still, it is not quite clear why both kinds of nitrogen, after being passed over the magnesium in Dewar's experiments, exhibited an almost similar alteration in their properties, independent of the appearance of a small quantity of hydrogen in them.

_Concluding Remarks_ (March 31, 1895).--The 'Comptes rendus' of the Paris Academy of Sciences of March 18, 1895, contains a memoir by Berthelot upon the reaction of argon with the vapour of benzene under the action of a silent discharge. In his experiments, Berthelot succeeded in treating 83 per cent. of the argon taken for the purpose, and supplied to him by Ramsay (37 c.c. in all). The composition of the product could not be determined owing to the small amount obtained, but in its outward appearance it quite resembled the product formed under similar conditions by nitrogen. This observation of the famous French chemist to some extent supports the supposition that argon is a polymerised variety of nitrogen whose molecule contains N_{3}, while ordinary nitrogen contains N_{2}. Should this supposition be eventually verified, the interest in argon will not only not lessen, but become greater. For this, however, we must wait for further observations and detailed experimental data from Rayleigh and Ramsay.

The latest information obtained by me from London is that Professor Ramsay, by treating cleveite (containing PbO, UO_{3}, Y_{2}O_{3}, &c.) with sulphuric acid, obtained argon, and, judging by the spectrum, helium also. The accumulation of similar data may, after detailed and diversified research, considerably increase the stock of chemical knowledge which, constantly widening, cannot be exhaustively treated in these 'Principles of Chemistry,' although very probably furnishing fresh proof of the 'periodicity of the elements.'

* * * * *

INDEX OF AUTHORITIES

Abasheff, i. 75 Abel, ii. 56, 326, 410 Acheson, ii. 107 Adie, ii. 186 Alexéeff, i. 75, 94 Alluard, i. 458 Amagat, i. 132, 135, 140 Amat, ii. 171 Ammermüller, i. 504 Ampère, i. 309 Andréeff, i. 251 Andrews, i. 136, 203 Angeli, i. 266 Ansdell, i. 451 Arfvedson, i. 575 Arrhenius, i. 89, 92, 389 Aschoff, ii. 313 Askenasy, i. 508 Aubel, ii. 45 Aubin, i. 238 Avdéeff, i. 618; ii. 484 Avogadro, i. 309

Babo, v., i. 93, 200, 203 Bach, i. 394 Bachmetieff, ii. 31 Baeyer, v., i. 507 Bagouski, i. 384 Bailey, i. 449; ii. 29 Baker, i. 318, 403 Balard, i. 480, 494, 495, 505 Ball, ii. 414 Bannoff, i. 506 Barfoed, ii. 53 Baroni, i. 331 Barreswill, ii. 282 Baudrimont, ii. 35 Baumé, i. 193 Baumgauer, ii. 20 Baumhauer, i. 495 Bayer, ii. 76, 159 Bazaroff, i. 409; ii. 24, 68, 486 Becher, i. 17 Becker, i. 16 Beckmann, i. 91, 496; ii. 156 Becquerel, i. 228; ii. 97, 220 Beilby, i. 71 Beilstein, i. 373; ii. 188 Beketoff, i. 120, 122, 124, 146, 403, 459, 466, 534, 541, 574, 577;