It is to be borne in mind, that the unities or starting-points for specific gravities and for atomic weights are essentially distinct. In the first case, the weights of the bodies are compared with the weight of an equal bulk of water; in the second instance, the combining numbers refer to a unit weight of hydrogen. Nevertheless, the relations observed between the specific gravities and the atomic weights are well marked in bodies of a like character. It has always been considered interesting to examine these relations in regard to Carbon, which has three well-characterised allotropic forms. The atomic volumes obtained by the above formula show no satisfactory relations between the numbers obtained for each of the states in which the element presents itself. Before we examine them in another way, it is desirable to obtain a mean specific gravity for the Diamond, Graphite, and Charcoal, as the recorded results of experiment show a considerable variation. 1. Diamond. The specific gravity of this gem is generally stated in elementary works to range from 3.5 to 3.55; but these numbers do not represent the mean of recorded experiments, as will be seen by the following table : Diamond in Hunterian Museum, Glasgow, 353 3.52 Thomson.1 Brazilian diamond, 3.44 Another variety of the same, 3.52 J } Brisson, 3 Mean specific gravity of a "beautiful collec tion of diamonds," 1 Thomson's Mineralogy, vol. i. p. 46. 2 Mohs' Mineralogy, vol. ii. p. 306. 3.461 3 Brisson, as quoted by Böttger, Specifiche Gewicht., p. 32. 4 Lowry, as quoted in Thomson's Mineralogy, vol. i. p. 46. 5 Dufrenoy, Compte Rendu, vol. xl. p. 3. 6 Grailich, Bull. Geol. [2], vol. xiii. p. 542. 7 Rivot, Ann. des Mines, vol. xiv. p. 423. 8 Jacquelain, Ann. de Ch. et Thys, [2], vol. xx. p. 459. 9 Henry's Mineralogy, vol. iv. p. 19. 10 Experiment made for this paper. If we reject the second Borneo diamond of Rivot, which has too low a specific gravity, we have a mean sp. gr. of 3.48, which is the same number as that found by Wilson Lowry for the mean specific gravity of his beautiful collection of crystallized diamonds" (Thomson's Mineralogy, vol. i. p. 46). It is to be expected that the experimental determination of the specific gravity of diamonds should be rather above than below the truth; for we are aware that they all leave a minute quantity of ash on burning, and that this ash, according to Petzhold, contains silica and iron. 2. Graphite. This variety of carbon is often impure, being not unfrequently contaminated with upwards of five per cent. of earthy impurities. Recorded specific gravities upon such impure specimens are of no value for the mean result as regards pure graphite. The following determinations are all those which I can find upon specimens which have been chemically examined to establish their purity: Natural graphite, Do. Do. Graphite of iron furnaces, Natural graphite, in fine crystalline plates, 2·14 Natural graphite, Karsten.3 Breikhaupt.3 Kengott.4 Fremy.5 2.35 Graham. 2.29 Gas carbon graphite, Mean sp. gr., It would have been interesting to have added to this list a determination of the specific gravity of Brodies' purified Ceylon graphite; but its minute division causes the air to adhere to it so tenaciously, that I have failed in getting any correct determinations of its density. 3. Charcoal. There are comparatively few determinations of the specific gravity of pure charcoal. It is in fact not so easy to obtain this substance. A specimen of charcoal from pure sugar, repeatedly calcined, and treated with chlorine to remove the last traces of hydrogen, and again calcined, gave me the sp. gr. 1.80; but bubbles of air still adhered to 1 Regnault, Ann. de Ch. et Thys., vol. Ivi. p. 37. 2 Schrader, Annals of Philosophy, vol. i. p. 299. 5 Traite de Chimie, vol. v. p. 518. 2 R it, although it was kept for several hours under a good air-pump. The following determinations are those recorded: 4. From the preceding data we take the mean specific gravity of the three varieties of carbon to be as follows: any simple relation to their atomic weight. The formula gives the following atomic volumes, taking £=12. These numbers do not bear to each other any simple relation. 6. If we now take the atomic weight of carbon (E=12), and then extract from it its square, cube, and fourth roots, numbers are obtained which bear a striking approximation to the mean specific gravity of the three forms of carbon: Sp. Gr. Diamond, 3.48 or 3.46 Graphite, 2.29 Charcoal, 1.88 In other words, if we raise the specific gravity of diamond to its second power, that of graphite to its third power, and that of charcoal to its fourth power, we obtain numbers closely approaching in each case to 12, the atomic weight of carbon. 1 Baudrimont, Traite de Chimie, vol. i. p. 511. 2 Colquhoun's Annals of Philosophy [2], vol. xii. p. 1. 3 Baudrimont, Traite de Chimie, vol. i. p. 514. 4 Regnault, Traite de Chimie, vol. i. p. 369. 5 Böttger, Specifiche Gewicht. 6 Experiment recorded above. Diamond, = 3.482 12.11 2.29312.00 1.884 12.49 = These approximations are remarkable, and the relations of the numbers are natural and simple. The differences between the mean experimental numbers and the corresponding roots of the atomic weight of carbon are not so great as the differences observed in the specific gravities of the same form of carbon. 7. It may be useful to condense into the form of a table the previous observations: These relations appear to be so simple, that it is scarcely possible to conceive that they may not have been described before; but I have been unable to find such descriptions. The nearest approach to it which I know, is the fact that Mr Hawksley, the engineer, stated to me, many years since, that he had brought under the attention of the late Mr Cooper the relation which seemed to subsist between the specific gravities of silver and gold and their atomic weights, this being approximatively the square root of their atomic weights, or of multiples of these numbers. But I cannot find any record either of Mr Hawksley's or Mr Cooper's views on the subject. 8. We know two other bodies besides carbon which possess diamond, graphite, and amorphous forms-viz., Silicon and Boron. If the same relation were observed between the specific gravities and atomic weights of these bodies, it would go far to establish as a law what, in an isolated case, might be due to a remarkable combination of chances. Unfortunately, we know only the specific gravity of the diamond forms of these elements :- Silicon diamond, Do., mean on six specimens, 2.49 2.48 Deville. Playfair. In quoting these results some explanation is necessary. In the original memoirs of Deville, it is left uncertain whether he examined the specific gravity of diamond or graphite silicon; and manuals of chemistry give it as the result due to the latter form; but from its coincidence with my own experiments on diamond silicon, it must unquestionably refer to that variety. Among the six specimens examined by myself, one preparation was in peculiarly fine crystals, and gave the sp. gr. 2.46; two out of the six specimens were prepared by Dr Matthiesien, and gave a mean sp. gr. of 2:47. The remainder were inferior samples, and probably contained zinc and other impurities. Professor Miller has kindly examined for me the specific gravity of a good specimen of graphite silicon in his possession (not analyzed), and found it to be 2.337. Deville gives as the specific gravity of boron diamond 2.68. The crystalline form of the boron diamond is the same as that of the carbon diamond, and similar relations seem to exist between the specific gravity and atomic weights. The atomic weight of boron is 7.2, viewing its oxide as corresponding to carbonic acid in composition. 7.2=2.683. sp. gr.=2.68. But the same relation would not appear to hold for silicon, which does not affect the like tendency to crystallize in the same forms as carbon and boron, although the relations between the numbers in its case also are in the same direction, and not devoid of simplicity. The atomic weight of silicon is 14·2. Si 14.2=2.42 Si/28.4 2.30 sp. gr. of diamond silicon, = 2:46 to 2·48. The differences exhibited in this case from the similar forms of Carbon and Boron are not sufficiently marked to throw doubt upon the relations as being due to some unexplained law. As an arithmetical probability, indeed, the discordance lessens the value of the But our chemical knowledge of testimony in the previous cases. the manner in which Silicon doubles and quadruples itself in the silicates, to unite with the same quantity of base, gives support to the idea that its atomic weights may be different in the various forms of the separate element. When we consider how much we multiply the errors of experiment in raising the observed specific gravities to the second, third, and fourth powers, it remains scarcely possible that the simple relations between them and the atomic weights, in the cases which I have pointed out, can be due to chance. I have purposely avoided any speculation as to the bearing which these relations may have on the molecular arrangement of the particles of the elements in their various forms, as I desire, in the first place, to submit the testimony on which the relations themselves are founded to the consideration of chemists. But it may be fairly asked, whether any similar relations exist between the specific gravities and atomic weights of the remaining solid or liquid non-metallic elements. I take the following mean specific gravity for bromine, iodine, sulphur, and selenium, and omit the only two remaining elements-phosphorus and tellurium—from the list, because they do not appear to yield relations at all analogous to those under consideration : |