cene molecules are greater, and the rates of vibration are slower, than those of the benzene molecules. Hence it would follow that the atomic vibrations which probably give rise to the observed selective absorption are closely dependent on the vibrations of the molecules as wholes. Now if a connection between the vibrations of molecules and the vibrations of parts of these molecules is established, and if this connection is elucidated by precise data, we shall certainly have made an important advance in solving the fundamental problem of chemistry, which is to trace the relations between the composition and the properties of bodies. A further step in this direction has been made by Abney and Festing', who, by mapping the absorption which occurs in the infra-red region of the spectrum, have been able to shew that there is a definite connection between the nature of the atomic groups in the molecules of many carboncompounds, and the vibrations of the rays stopped by these compounds. 149. The preceding paragraphs of this section will, I think, shew how promising of important results is the application of optical methods to the problems of chemical statics. That a relationship exists between refractive power and molecular structure, and also between rotatory power and molecular structure, has been established. In the hypothesis of Brühl, which connects the former physical constant at once with the valencies of atoms and with the distribution of atomic interactions, and in that of van't Hoff, which has a more kinetical aspect than the hypotheses regarding molecular composition at present dominant in chemistry, we have guides to future research. But much more data, dealing with groups of allied compounds, must be brought together before either of these hypotheses can be fully tested". 1 Proc. R. S. 31. 416, and Phil. Trans. for 1881.7887. 2 Reference may here be made to a paper by G. Krüss [Ber. 15. 1243, and 16. 2051] on an optical method for determining whether or not chemical action has occurred between two substances in solution, all the possible products of the reaction being also soluble under the experimental conditions. The method consists, essentially, in comparing the sums of the absorption-spectra of the original liquids with the absorption-spectrum of a mixture of these liquids, SECTION III. Methods based on determinations of the constant, formula-weight, specific-gravity 150. The quotient obtained by dividing the formulaweight by the specific gravity of a compound (referred to water at 4°) is generally called the specific volume of that compound. The term specific volume, however, evidently expresses the relative volume of unit weight of the substance. The quotient in question is sometimes called the molecular volume of the compound formulated. This expression strictly interpreted implies that the formula-weight is identical with the molecular weight, and that the specific gravity and formula-weight are expressed in terms of the same standard. formula-weight The value of is equal to the product of spespec. gravity cific volume multiplied into molecular weight, assuming the latter to be the same as the formula-weight; or we may say that, if the weight expressed by the formula is taken in grams, formula-weight the quotient represents the number of cubic spec. gravity centimetres occupied by an amount of the substance in grams proportional to its molecular weight. Now we can determine the molecular weights of gaseous compounds only: if the specific gravities of these compounds are referred to hydrogen molecular weight as unity, then, = c, and c = 2. Never spec. gravity 1 It may be well to gather together here references to the most important articles and papers on the subject of this section :-KOPP, Annalen 96. 153, 303; 100. 19, &c. BUFF, Annalen Supplbd. 4. 129, and Ber. 4. 647. THORPE, C. S. Journal Trans. for 1880. 141, 327. L. MEYER, Annalen Supplbd. 5. 129; also Die modernen Theorien (4th Ed.), 284-292. ELSÄSSER, Annalen 218. 302. WEGER, Annalen 221. 61. WATTS's Dict.; 1. 440 et seq. and (more especially) 3rd Supplt. 2117 et seq. RAMSAY, C. S. Journal Trans. for 1879. 463; do. for 1881. 49. 66. LOSSEN, Annalen 214. 81. Compare also SCHIFF, Ber. 14. 2761; 15. 1270; Annalen 220. 71, 278. SCHALFEJEW, Ber. 15. 2209; 16. 1853. See also O. E. Meyer's Die Kinetische Theorie der Gase, 216–221. KRAFFT, Ber. 15. 1687. WILSON, Proc. R. S. 32. 457. theless, if the quotient formula-weight is obtained for a number of liquid compounds, we shall have a series of comparable values, which,—if formula-weight of liquid is a simple multiple of molecular weight of gas,-represent the volumes occupied by quantities of various liquid compounds proportional to the molecular weights of the same compounds in the state of gases. The meaning to be attached to the expression 'volume occupied by a quantity proportional to molecular weight' will be discussed in paragraph 156. The name atomic volume is generally applied to the quoatomic weight tient (water = 1). spec. gravity of liquid element The determinations of the specific gravities of liquids necessary for finding values for the quotient we are discussing, should be made under comparable conditions as regards pressure. This condition is fairly fulfilled by determining the specific gravities at the boiling points of the liquids'. formula-weight of liquid compound 151. Let the quotient spec. gravity referred to water at 4° be expressed by the symbol (V). Then the value of (V) for a compound is said to be equal to the sum of the values of (V) for the elementary atoms which form the molecule of that compound. But has each elementary atom a constant value? For many carbon compounds Kopp has shewn that (V) C2H2O2=(x.11)+(y. 5°5)+(≈.7·8). But in some cases the observed value of (V) does not agree with that calculated by this formula; thus Aldehyde CHO: calculated (V)=(2.11)+(4.55)+78=518: observed (V) observed (V) diff. =56'5. +47 =63'5. +39 Acetic acid C,H,O,: calculated (V)=(2.11)+(4.5'5)+(2.7.8)=59'6: 1 Full details regarding the methods for accomplishing this will be found in Thorpe's paper, loc. cit.; see also Ramsay (loc. cit.) and Schiff (loc. cit.). The value of (V) for a compound C,H,O, is conditioned, according to Kopp, by the value of (V) for the oxygen atom, or atoms, in the molecule. Kopp gives the following two values, according as the oxygen atom acts as a monovalent or divalent atom in the given molecule'. (V) O1=12′2; (V)0"=7·8. Applying these values to the case of aldehyde, we have (V)H,C-C-O=(2.11)+(4-5′5)+12*2=56'2; H a result which agrees very closely with the observed value, viz. 565. For acetic acid we have (V) HC-C = (2.11) + (4.5°5) + 7·8 + 12′2 = 64°0: observed = 63'5. он Or, once more, for acetone and its isomeride allylic alcohol, (1) (√) H ̧C—C—CH1=(3.11)+(6.5′5)+12′2=78'2: observed = 78'0; (2) (V) H2C—C—C—OH=(3.11)+(6.5°5)+7·8=73·8: H H2 Instead of assigning two values to the oxygen atoms in compounds of the form C,H,O,, it would probably be better to employ the value, (V) CO = 23·2 (i.e. 11+12′2), which attributes the influence on the total value of (V) due to the presence of the group CO to both the atoms which comprise this group. Schiff (loc. cit.) concludes that the value of (V) O1 varies according to the nature and arrangement of all the con 1 Kopp used the expression 'oxygen within the radicle' as synonymous with what is now called divalent (singly-linked) oxygen atoms; and 'oxygen without the radicle' as synonymous with monovalent (doubly-linked) oxygen atoms. stituents of the molecule; and also, that the value of (V) X − C – O is always greater than that of (V) C−0−x, when X represents a radicle. Kopp deduced two values for (V)S; thus (V)S1 = 28·6, (V)S" = 22'61: but only one value for (V)C, and one for (V)H, and (V)Cl. Many, and very varying values, have been found by different observers for (V)N: thus Kopp assigns the value 2.3 to (V)N when N occurs in amines, and 17 when N occurs in CN and in some nitro-compounds; Ramsay gives (V)N = 3.6 in amines, = 9'0 in pyridine, lutidine, &c., and 7 in aniline, toluidine and dimethylaniline. = 152. If the influence exerted by the oxygen in a carbon compound on the value of (V) for that compound, varies, according to the actual valencies of the oxygen atoms in the molecule, it appears probable that the total value of (V) will also depend on the actual valencies of the carbon atoms in the molecule. Buff thought that his determinations shewed that the value of (V) for compounds containing trivalent (doubly-linked) carbon atoms is greater than the value calculated on the assumption that (V)C = (V)C1V = 12′2. Thus, (1) Dichlorethylene Cl2- C-CH2, (V)=79′9: (V) calculated=786; diff.=1*3+. (2) Carbon tetrachloride Cl, CC Cl2, (V)=115'4: (3) Amylene H,CIV (V) calculated=113'2; diff.=2'2+. сС" — С" — СTMH2, (V)=112: (V) calculated=110; diff.=2'0+. H (4) Valerylene H ̧СTM — СTM — С" — С"—С"H2, (V)=104′0: H No trustworthy conclusions regarding the values to be assigned to (V)CII or (V)CIV can however be drawn from 1 See also Ramsay, C. S. Journal Trans. for 1879. 471-2. 2 Annalen, Supplbd. 4. 143 et seq. M. C. 22 |