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Chemical Notes

Nature volume 31, page 204 | Download Citation



ATTENTION was lately drawn in these Notes to Schiff's recent researches on the connections between the capillary coefficients of various liquid carbon compounds and the structure of the molecules of these compounds (see also NATURE, vol. xxx. p. 618). The same subject has very recently been examined by J. Traube (Ber. xvii. 2294). Traube thinks that the differences between the various capillary elevations observed by Scruff are too small to allow of trustworthy conclusions being drawn: he has therefore undertaken a series of observations with aqueous solutions of various classes of carbon compounds. Inasmuch as the capillary elevation of water in a tube of .34 mm. radius is about 41.5 mm., while that of most liquid carbon compounds does not exceed 25mm., Traube concluded that there will probably be well-marked differences between the capillary elevations of aqueous solutions, and mixtures of aqueous solutions, of definite concentration, of various compounds of carbon. The height in capillary tubes was determined for each solution for varying degrees of concentration, and the results are stated for equal weights of compounds in equal volumes of solution. From these results Traube draws the conclusions:—(1) The capillary elevation of the solution of a compound decreases as concentration increases; the differences of elevation are not equal for equal increases in concentration. (2) The capillary elevations decrease in a homologous series of carbon compounds as molecular weight increases. (3) Isomeric compounds in solutions of equal concentration do not always exhibit equal capillary elevations. SchifFs generalisation, that the number of molecules of isomerides raised by capillary action is equal, does not hold good for aqueous solutions of isomerides. As in Traube's experiments the liquids examined were of equal concentration, it follows that the ratios of the capillary elevations are equal to the ratios of the masses of the dissolved compounds raised in the capillary tubes. Calling the capillary elevation h, and the specific gravity of the solution s, Traube considers the product hs, which he calls the capillary coefficient of the solution. The value of h is conditioned by the chemical constitution of the compounds examined. If m = molecular weight of compound in solution, then the difference between (h/m for solutions of two compounds, within m certain limits of concentration, is a constant which depends only on the relative concentrations of the two solutions. The values of (h/m for an homologous series, dealing with solutions containing equal masses of the compounds in equal volumes, are referred to the value of (h/m for the first member of the series, and the differences thus obtained, when calculated for a tube 1 mm. radius, are called the specific capillary constants of the compounds in the series. The values of this quantity are almost wholly dependent on the nature of the solution, perhaps only on the nature of the dissolved substance, and are independent, within certain limits, for each homologous series, of the absolute concentration of the solutions, and are scarcely, if at all, dependent on temperature. Traube thinks he is justified from his experimental results in concluding that the differences between the capillary elevations of the solutions of two analogous compounds are in the same ratio as the molecular weights of the compounds. Thus, let hα and hα1 represent the capillary elevations of two solutions, of different concentrations, of the compound with molecular weight m and let hβ and hβ1 represent the capillary elevations of two solutions, of the same concentration as those of the former compound, of an analogous compound with molecular weight m1 Then, according to Traube,

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