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represented by the symbols H,, Cly, and N, respectively. These symbols represent weights of equal volumes of the three elements; if one of these weights be taken as the unit, the other weights are evidently the weights of the molecules of the gases in question referred to this standard, because equal volumes contain equal numbers of molecules, and therefore 'the masses of the two kinds of molecules are in the 'same ratio as the densities of the gases to which they belong.'

Hydrogen is the universally adopted standard of reference for molecular weights.

The modern molecular theory of matter is not identical with the atomic theory of Dalton; it is based on evidence of a different kind, it is essentially a physical and dynamical theory, although strengthened by chemical arguments. The atomic theory of modern chemistry may be regarded as growing out of the application of reasoning founded on chemical facts to the molecular theory of matter.

Assuming 'Avogadro's law' and remembering that the hydrogen molecule divides into two parts in many chemical changes, we arrive at the practical definition of molecular weight.

The molecular weight of a gas is the weight of that volume thereof which is equal to the volume occupied by two parts by weight of hydrogen.

In determining the specific gravity of a gas it is easier, and less liable to error, to find the weight of the vessel filled with air than with hydrogen; the result is therefore stated as specific gravity referred to air as unity. Now the specific gravity of hydrogen is '06926 [air=1]; the molecular weight required is specific gravity referred to hydrogen as 2, hence if M=molecular weight, and d=specific gravity referred to

2.d air as unity, M=

28.87.d. Hence the practical rule

'06926 for determining the molecular weight of a gas :

Find the specific gravity, i.e. the ratio between the weights of equal volumes of the gas and air under the same conditions of temperature and pressure, and multiply this by 28.87.


15. The following table presents the results hitherto obtained regarding the molecular weights of elementary gases.

[The numbers in column v are not always exactly equal to the products obtained in column iv; for an explanation see pp. 34-35.)

Molecular weights of elementary Gases.

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Hydrogen 2 Nitrogen 3 Oxygen

(ozone) Sulphur

about 1400°


28.04 3194 3192











'oбo26 0*9713 I'106 1'10563 1658 2'23 2'24 2:17 2993 662 2:45 2.61 2:44 394 4:35 4:50 102 1065 5:54




10 Chlorine
12 Cadmium
13 Phosphorus
15 Arsenic
17 Bromine





20 Selenion


about 1400


about 1000
about 1200°
about 1000

about 1000°



100° about 1500° about 1400° about 1000®

860° about 1000


250°— 450°


447° about 1000


6650 about 1100° about 1500° about 1400°

644 64:6 62.6 84.6 1911 70*73 75-35 70*72 1137 1256 1299 294'5 3074 1599 155-3 1179 161'1 1839 2214 200'93 2015 2030 1934 254'0 25197 251'2 2517 2552 246.8 1694 1374 2621


? 1576

? 2364



4:43 5*68 6'37 7:67 6.96 6.98 7'03 697 8.8

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27 Iodine

28 29

872 8.70 8.72 8:84

30 31


31s 32

8.55 5:87 476


? [? 126.53]


31 Tellurium


1 REGNAULT, Compl. rend. 20. 975. Ber. 12. 1426. 4 REGNAULT, loc. cit.

: Ibid., loc. cit. 3 V. MEYER,

5 SORET, Compt. rend. 61.

The specific gravities of the vapours of potassium and sodium have been determined by Dewar and Dittmar (Chem. News, 27. 121), and by Dewar and Scott (ib. 40. 293): but the numbers are not given in this table because V. Meyer has shewn that the process made use of was not trustworthy (Ber. 13. 391).

16. So many determinations of molecular weights of compound gases have been made that an enumeration of all the results would be perplexing, and of no special value. The method is applicable to elements and compounds alike. The following numbers are given here as they illustrate a point of general importance.

Specific gravities of certain compound gases.


Sp. gr.



Phosphorus pentachloride 5'08


369 290°

Nitrogen tetroxide




83° 1950





941 and 64. 904. 6 and 7 Deville and Troost, Compt. rend. 56. 891. 8 V. Meyer, Ber. 12.

8a Troost, Compt. rend. 95. 30. 9 DUMAS, Ann. Chim. Phys. (2) 50. 170. 10 LUDWIG, Ber. 1. 232. 11 V. MEYER, Ber. 13. 400.

11. Ibid. do. 16. 2773 (mean of 5 experiments). 12 Deville and Troost, Compt. rend. 49. 2 39.

13 and 14 Ibid. do., 56. 891. 15 Ibid., loc. cit. 16 MITSCHERLICH, Annalen 12. 159. 17 Ibid., loc. cit. 18 V. Meyer, Ber. 13. 406. 19 Crafts, Compt. rend. 90. 183 20, 21 and 22 DEVILLE and TROOST, loc. cit.

23 V. MEYER, Ber. 13. 1107 and 1110 (mean of 6 experiments). 24 DUMAS, Ann. Chim. Phys. (2) 33. 337. 25 MITSCHERLICH, loc. cit. 26 BINEAU, Compt. rend. 49. 799

27 V. MEYER, and Meier and Crafts, Bcr. 13. 868 (mean of 7 experiments). 28 DUMAS, loc. cit. 29 and 30 DEVILLE and TROOST, loc. cit. 31 V. Meyer, Ber. 13. 396. 31 a TROOST, Compt. rend. 95. 30. 32 V. MEYER, Ber. 13. 1115.

33 Ibid. do. 13. JO10. 34 DEVILLE and Troost, loc. cit.

Note to preceiling table. The expression 's gravity of a gas' will be employed to denote the specific gravity referred to air as unity: the expression *vapour density of a substance' to denote the specific gravity of a substance in the gaseous state referred to hydrogen as unity.

350° 450°


Sp. gr.

Ferric chloride


Arsenious oxide



1378 1400° From these numbers, and from those of the previous table, it is apparent that the specific gravities of certain gaseselementary and compound alike—decrease as the temperature increases, while in the case of other gases the density is practically independent of the temperature; a limiting value is however generally found for the specific gravity of a gas.

It would therefore appear that a chemical substance may have more than one molecular weight; but if the molecule is the smallest part of a substance which exhibits the characteristic properties of that substance, this is equivalent to saying that certain substances when heated may pass through a succession of changes, each phase being marked by the existence of a distinct kind of matter. More accurate experiment has shewn that the vapours of phosphorus pentachloride and nitrogen tetroxide, at high temperatures, are mixtures of phosphorus trichloride and chlorine, and of nitrogen tetroxide and nitrogen dioxide (N,O, and NO,) respectively, so that at these temperatures we have to deal not with homogeneous vapours, but with mixtures of different gases, varying in composition at different moments. The connection existing between temperature and the densities of gaseous elements and compounds will be examined in more detail in a future chapter? (see Book 11.).

The practical outcome of these considerations is that in determining a molecular weight the gas must be proved to be really a homogeneous substance, and not a mixture pro

1 Avogadro's law may be deduced from the molecular theory of matter, but inasmuch as this theory is based upon more or less inexact hypotheses, and is as yet but in an early stage of development, inasmuch also as the deductions made from it concerning gaseous laws are strictly applicable only to “perfect gases,' it follows that Avogadro's law cannot be regarded, at present, as absolutely true. The laws of Boyle and of Charles, which are also deducible from the molecular theory, do not give a complete account of the relations of gases to temperature and pressure. M. C.


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duced by the decomposing action of heat on the original substance; and, further, that the value obtained for the specific gravity must be constant throughout a considerable range of temperature.

17. In determining the density of a gas, especially if at a somewhat high temperature, many sources of error are present; the result cannot therefore be more than moderately accurate! But experimental errors are more easily

'. avoided in the determination of the combining weight of an element, that is, the quantity of the element found in combination with one part by weight of hydrogen, 7-98 parts by weight of oxygen, or 35-37 parts by weight of chlorine. Now it is evident that the molecular weight of an element must be equal to, or a multiple of its combining weight, and the molecular weight of a compound must be equal to the sum, or to a multiple of the sum of the combining weights of its constituent elements. Hence if the combining weight, and the specific gravity in the gaseous state of an element are carefully determined, we have the necessary data for an accurate determination of the molecular weight of that element; the combining weight being an accurately determined number, and the specific gravity deciding what multiple of that number represents the molecular weight. So also the data required for an accurate determination of the molecular weight of a compound are, the combining weights of the constituent elements, and the specific gravity of the com

1 Dumas' method for determining vapour densities is described in Ann. Chim. Phys. [2] 33. 337; Gay Lussac's in Biot's Traité de Phys. 1. 291; Hofmann's in Ber. 1. 198; and Victor Meyer's Ber. 11. 1868 and 2253. For criticisms on, and modifications of Meyer's method see Ber. 12. 609 and u12: 13. 401, 851,991, 1079, 1185, and 2019: 14. 1727: and 15. 137, 1161 and 2775: (in the last paper by V. Meyer (Ber. 15. 2775] will be found an interesting and valuable criticism of the various methods for finding the Sp. Grs. of gases). See also Ber. 16. 1051; also C. S. Journal Trans. for 1880. 491. Modifications of Dumas' method are described by Bunsen, see Gasometrische Methoden, 2nd ed. (1877), p. 172: also by Petterson and Ekstrand, Ber. 13. 1191: and especially by Pawlewski, Ber. 16. 1293. Thorpe [C. S. Journal Trans. for 1880. 147-150] has described a very complete method based on Hofmann's process. V. Meyer (Ber. 9. 1260: and 10. 2068] has described a method based on the displacement of mercury.

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