these data, because when we tabulate the values of (V) for a number of hydrocarbons we find no apparent regular connection between these values and the valencies of the carbon atoms. Thus, (1) Hexane H,C—(CH2)4—CH3 (V)=140: (V) calculated=143. (2) Diallyl H,C-c-c-c—C—CH2 (V)=126.8: H H2 H2 H If we associate the increase in the value of (V) for diallyl over the calculated value, with the presence of trivalent carbon atoms, then we must conclude, that in the molecule CH, the presence of trivalent carbon atoms is connected with a decrease in the calculated value of (V), or that all the carbon atoms in this molecule are tetravalent. 153. But not only may the values to be assigned to carbon and oxygen atoms, in determining the total value of (V) for a carbon compound, vary according to the actual valencies of these atoms in the molecule of the compound in question, but also, apparently, in accordance with the distribution of the interatomic reactions in molecules wherein all the carbon atoms are tetravalent, and all the oxygen atoms divalent. Thorpe (loc. cit.) has given some examples of such variations; but Zander' has extended the number of examples considerably. Thus a comparison of (V) for propyl and isopropyl compounds shews that the normal compounds always exhibit a smaller value than the iso-compounds: but the molecules of both classes of compounds contain only tetravalent carbon atoms1. 2n+1 2 Lossen has collected the most trustworthy data bearing on the question as to whether or not a constant value can be assigned to (V)CH, Kopp gave 22 as the mean value for this group. Lossen shews that the differences between the values of (V) for successive homologues of the acid series CnH2+CO2H nearly agree with the differences calculated on the basis of (V)CH, = 22; but that in the series of alcohols CH2nt 【2n+1CH2OH the value of (V)CH, varies from 18.7 to 21, assuming that the other atoms exert a constant influence on the total value of (V). Apparently then a variable value must be assigned to (V)CV, or to (V)H, or to both of these quantities. Some light is thrown on this point by Zander's comparison (loc. cit.) of (V) for propyl, isopropyl, and allyl compounds, which leads to the conclusion that the difference between (V) for a normal propyl and the corresponding allyl compound, i.e. between two compounds differing in composition by H,, varies from 5'7 to 8.9 (having a mean value of 7.1): hence, if we assume that the difference in question is wholly due to the difference in empirical composition, we appear forced to conclude that the value of the influence exerted on (V) by the monovalent atom H is variable3. Thorpe (loc. cit.) got these results for compounds containing only tetravalent carbon atoms in their molecules: 1 See also Brown, Proc. R. S. 26. 238. Also Elsässer, Annalen 218. 302. 2 Annalen 214. 81 et seq. 3 Besides the empirical difference of H2, there is a difference in the actual valencies of some of the carbon atoms in propyl and allyl compounds; thus, normal propylic alcohol H,C-C-C-OH, and allylic alcohol H2 H2 = = H2C-CC-OH. See also Weger, Annalen 221. 61, who gets different H H2 values for (V) CH, in different series of compounds. See Ber. 16. 2458, where Kopp reminds us that this number was given by him as a mean value, and nothing more. Schiff (Annalen 220. 286, and 291) concludes that (V)C almost certainly varies according to the nature and the arrangement of the constituents of the molecule in which C occurs. and (V) H=5'5) H2CCl2 (V)= 65'12; hence (V) Cl=216; (assuming (V) C=11, Taking the mean value for (V)Cl, viz. 22°7, and applying this to calculate the values of (V) for each of the preceding compounds, we have Hence the value of (V)Cl appears to be variable. This is more strikingly illustrated by Stædel's comparison' of the differences in the values of (V), and also the differences in the boiling points, at various pressures, of chlorine compounds derived from C,H,. The differences in (V), and also in B.P. between the following pairs of compounds, viz. CIH,C-CH,Cl and HC-CH2Cl, CIH,C-CCl, and HC — CCl3, express differences corresponding with change of CH, into CH,Cl, i.e. with the introduction of the first chlorine atom in place of an atom of hydrogen into the hydrocarbon residue CH,. The differences in the values of the same quantities between the following pairs of compounds, viz. Cl2HC-CH3 and H2CIC - CH 3, Cl2HC — CH2Cl and H2CIC — CH2Cl, express differences corresponding with the introduction of the second chlorine atom (in place of an atom of hydrogen) into the residue CH,. 1 Ber. 15. 2559. And lastly, by comparing (V) and B.P. for the following pairs of compounds, viz.: Cl2C - CH, and Cl2HC - CH3, ClC-CH,Cl and Cl2HC-CH2Cl, Cl2C - CHCl, and Cl2HC-CHC12, the differences corresponding with the introduction of the for the first chlorine atom (V)=14'20: B.P. = 56°22 ; Hence each chlorine atom has a different volume-value' and a different 'boiling-point-value.' If we choose to attribute the observed differences to the carbonaceous parts of the molecules, i.e. to C2H ̧ in C2HCl, to CH ̧ in CH ̧Cl ̧, &c., we seem still obliged to admit that carbon and hydrogen atoms have varying 'volume-values', and varying 'boilingpoint-values', in the molecules formulated. 154. The remark made in paragraph 151 that the value of (V) for a compound is equal to the sum of the values of (V) for each of the elementary atoms in the molecule of that compound, must evidently be supplemented by the statement, that in the case of carbon compounds, at any rate, the value of (V) is not constant for C or O, and probably not for Hor Cl, but varies in accordance with (1) the actual valencies of the former pair of atoms, and (2) the distribution of all the atomic interactions in the molecule. The precise character of the connection between the values of (V) for C, O, H, and Cl, and the valencies on the one hand, and the nature of the atoms (or atomic groups) in direct union within any molecule on the other hand, cannot be ascertained until much more experimental data has been accumulated'. The known data regarding the values of (V) cannot therefore be 1 It is pointed out by Lossen (loc. cit.) that careful determination of (V) for many series of carbon compounds and for many individuals in each series, are now required. applied in other than a very tentative way to the selection of one from among several possible structural formulæ1. 155. The values of (V) for many solid compounds have been compared, and attempts have been made to generalise the relations between these values; but, as might be expected from considering how little comparable are the conditions under which the densities of solids have been determined, the conclusions are either vague and difficult of precise application, or represent only interesting relations between certain numbers, without much, if any, connection with chemical facts. By considering the difference between (V)MO and (V)M, a fairly constant value for (V)O in the oxides is sometimes obtained thus for PbO and Fe,O,, the difference in question is about 55. But in other oxides the value of (V)O appears to be very variable; thus, (V) CuO-(V) Cu=5'1; but (V) Cu2O − (V) Cu1=10'5. Brauner and Watts have drawn the following conclusions from comparisons of (V)MO and (V)M for different series of oxides. (1) In strongly basic oxides the value of (V)O is negative; the more basic the oxide, and the greater the value of (V)M in the oxide, the more negative is the value of (V)O. (2) In oxides of heavy metals and non-metals the value of (V)O is positive. (3) In oxides of the earth metals the value of (V)O is nil. The values of (V) for isomorphous compounds generally vary little; thus, (V) MgO. Al2O3=41'4 (V) ZnO. Fe2O3=47°0 The greater the agreement between the angles of crystals 1 An illustration of the difficulties which are met with, and of the uncertain nature of the results obtained, is furnished by the contradictory conclusions of Thorpe (see Watts's Dict. 3rd Supplt. 2117-18) and of Masson and Ramsay (see C. S. Journal Trans. for 1881, 51 et seq.) regarding the structural formula of POCl3. Thorpe concludes that the formula ought to be written Cl=P-O-Cl, Masson and Ramsay think that Cl, PO more nearly represents the facts. 2 Phil. Mag. [5] 11. 60. |