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numbers. Thus, comparing the observed and calculated heats of combustion (because these are the bases for calculating heats of formation) of benzene and dipropargyl, we have this result.

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I. HEAT OF COMBUSTION OF BENZENE (C.H.). On the assumption that each carbon On the assumption that each carbon

atom is trivalent (or, that the atom is tetravalent (or, that the molecule contains 3 'double molecule contains 9 single bonds' and 3 'single bonds' bonds' between carbon atoms).

between carbon atoms). Calculated. Difference. Observed. Calculated. Difference. Observed. 844,500

- 38,700 805,800 (1) 800,400 + 5,400 805,800 (1) 844,500 - 56,500 788,000 (2) 800,400 - 12,400 788,000 (2)

(1) Earlier, (2) later observations.

=776,000 >77,600

II. HEAT OF COMBUSTION OF BENZENE AND DIPROPARGYL (C2H).
Berthelot's numbers.
Observed Difference.

Thomsen's numbers. dipropargyl=853,600

883,200=dipropargyl benzene

. 95,200

788,000=benzene Calculated difference, (1) on the assumption that each carbon atom in the

benzene molecule is tetravalent=88,200. Calculated difference (2) on the assumption that each carbon atom in the

benzene molecule is trivalent=44,100.

The numbers representing the heats of combustion of dipropargyl and benzene obtained by Berthelot, are considerably greater than those obtained by Thomsen ; the difference amounts, in the former case, to 29,600, and in the latter case to 12,000 units.

It would evidently be absurd to draw any precise conclusions regarding the structure of the molecules of benzene and dipropargyl from these results?.

Two general conclusions may, I think, be drawn from Thomsen's investigation ; (1) that the energy-changes attending the formation of isomeric molecules are correlated, not only with the valencies of the constituent atoms, but also with the distribution of the atomic interactions; (2) that the use of

1 For a criticism of Thomsen's conclusions regarding the structure of hydrocarbons see Mendelejeff, Ber. 15. 1555; or C. S. Journal, Abstracts for 1882, 916.

? Lothar Meyer, Die Modernen Theorien der Chemie, 424, puts the conclusion to be drawn regarding the “ linking' of two carbon atoms thus, “what we call

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that nomenclature which is founded on the hypothesis of bonds ought to be abandoned'.

135. A few generalisations have been established regarding the connection between the structure and the boiling point of carbon compounds. Thus the difference between the boiling points of two consecutive members of an homologous series of carbon compounds is frequently about 19° : but the numbers actually obtained shew that variations in the boiling points are connected with variations other than those of molecular weight. Goldstein' attempts to shew that the proportion between the numbers of hydrogen and carbon atoms, besides the total number of these atoms, influences the boiling points of the members of an homologous series. Hydrocarbons of analogous constitution must be compared, i e. normal hydrocarbons must be compared with normal, e.g.CH,-CH,-CH,-CH with CH,-CH,-CH,-CH-CH,; or iso-with iso-hydrocarbons, e.g. CH(CHI) - CH - CH, with CH (CH)-CH-CH-CH,; nor can the differences between the boiling points of normal, be compared with the differences between the boiling points of iso-hydrocarbons.

Goldstein investigates the change of boiling point in the series of normal paraffins : i.e. hydrocarbons of the form CH,- (CH). -CH, [or CH, -CHR' - CH). He gives the formula

380 .

n(n+1) where B. P. = boiling point required, b. p. = boiling point of the paraffin containing CH, less than that whose B. P. is required, and n= number of atoms of carbon in the molecule of the paraffin whose B. P. is known. Thus, the boiling point of C:H,, (i.e. CH, (CH2), CH) is 39°0; required the boiling point of C.H. (i.e. CH,(CH),CH,). double, or triple, linking of atoms, does not consist of a repetition of the process which we call single linking."

1 See ante, chapter 11. section iv. par. 84.

2 Ber. 12. 689: also abstract of paper in Russian, C. S. Journal Abstracts for 1882, 374,

2

B. P. = b. p. + (19+

=

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12

3

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14

+(19+

380 B.P. required = 39+(19+

n(n+1)

380
= 39+19+

30
= 39+19+12:06
=70°•66.

B.P. observed=70°:6. Goldstein calculated the B. P. of normal heptane (C,H..) to be 980:65; shortly after this, the paraffin was obtained in quantity by Thorpe, and the boiling point was found to be 98°5.

The same formula appears to hold good for determining the difference between the boiling points of any two consecutive iso-paraffins belonging to the form CH (CH),-(CH). -CH, Thus,

B.P. Difference. (a) CH(CH3)2-CH2-CH, 30°05 (6) CH(CH3), -(CH.),-ch, 62031o5...calculated difference = 31°66.

If this is so, it follows that the difference between the boiling point of a normal – and its corresponding iso-paraffin (of this form) must be the same whatever be the molecular weight of the two isomerides. Experiment, so far as it has gone, seems to confirm this result; thus,

Difference between B.P. of normal

and iso-paraffin. CH

8°:5 C2H14

8°.6 C,H16

8005 Kahlbaum has made some determinations of the ratio between the change in the boiling points of ethylic alcohols and acetic acids, and the diminution of pressure, and has concluded that a definite relation exists between at least the empirical formula of a compound and the ratio in question.

We have very little precise knowledge regarding the boiling points of isomeric hydrocarbons. From the data accumulated it has been concluded, that, of two or more isomeric hydrocarbons, that one has the lowest boiling point, the molecule of which is characterised by containing the greatest number of 'side chains'? Thus

i Ber. 16. 2476: 17. 1245, and 1263.

2 For data see Naumann, loc. cit. pp. 168–172. M. C.

20

Formula.

12

1

39°

Pentane (C3H12).

B.P. (a) normal :-CH(CH2)2 -CH (6) isopropyl-methylmethane :-CH2-CH(CH3)2 - CH, 30°:5 (c) tetramethylmethane :-C(CH3)4

9°:5 Hexane (C H14. (a) normal :-CH(CH2) CH3

70°5 (6) isopropyl-ethylmethane :-CH-CH(CH3)2-C,H, 62° (c) di-isopropyl :-CH(CH3)2 -CH(CH3)2

58° (d) trimethyl-ethylmethane :-C(CH(C,H3)

43-48° 136. In this section I have tried to trace some of the connections between the results of thermal measurements of chemical phenomena and the statical aspects of these phenomena. We have found that every chemical phenomenon is a complex occurrence, and that it is almost impossible fully to differentiate those portions which would more appropriately be called physical, from those which are undoubtedly chemical. We have also found that thermal measurements, being essentially measurements of changes of energy, are intimately connected with problems belonging to chemical kinetics, and that until we have some precise knowledge regarding chemical affinity we are not in a position fully to discuss the data of thermal chemistry.

SECTION II. Optical Methods.

137. In this section I wish to give some account of the attempts which have been made to elucidate the relations existing between (1) the refractive powers, (2) the power of rotating a ray of polarised light, and (3) the absorption spectra, and the composition of certain chemical compounds. The subject is more limited than that considered in the first section of the present chapter; it belongs, more completely than thermal chemistry, to the domain of chemical statics, although like other questions in chemical science, it is under certain aspects best considered from a kinetical point of view.

138. Let a ray of light pass from air into a liquid medium denser than air ; let the angle of incidence = i, and the angle

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dium.

Let the light employed consist only of light of one wave-length, and let the liquid medium consist of a single

definite chemical compound, then the quantity (7")

was

d called by Gladstone and Dale' the specific refractive energy of the liquid examined, (d= density of the liquid referred to water as unity). Landolt’ called the product obtained by

d

multiplying (2-) into the molecular weight of the liquid, i.e.(*72-) M, the refraction equivalent of the liquid compound

The quantity (2)

in question.

was said by Gladstone and Dale to

d be independent of temperature”.

The refraction-equivalent of the molecule of a chemical compound is generally said to be the sum of the refractionequivalents of the atoms which compose the molecule: or the refraction-equivalent of a mixture is the sum of the equivalents of its components“.

But objection has been taken by Wiedemann to the use of the constant

M in attempts to trace connections

d between the composition and the optical properties of compounds. Any relations which appear to exist between molecular composition and physical properties must, it is urged, be formal rather than real relations, as long as the properties of the molecule are assumed to be the sum of the properties of the atoms. Physical constants ought to be employed which determine the properties of atoms.

( (7)

1 Proc. R. S. 12. 448, and Phil. Trans. 153. 317. · Pogg. Ann. 122. 545; and 123. 595. 3 See Proc. R. S. 18. 49; and also Phil. Trans. 160. 9. * See especially Landolt, Pogg. Ann. 123. 623. 5 Ber. 16. 467.

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