Page:Text-book of Electrochemistry.djvu/68

 The tube contains, besides the liquid to be examined, the thermometer G and a column of glass beads, 2 to 3 cm. high, which causes the iKjUing to be more even. This vessel ia surrounded by a vapour-mantle, D, made of glass, porcelain, or metal, which ia half filled with the same solvent (or solution) as is contained in A. The tulje and the mantle are separated below by a ring of asbestos, and both are provided with air-conden- sers, or, if the solvent be very volatile, with small Liebig condensers, C and F. The apparatus rests on an asbestos stand, fitted with funnels, so that the heat can be easily regulated. With this arrangement Beckmann has succeeded in maintain- ing the boiling point con- p,a_ ig stant mthin a few thou- sandths of a degree, a result which had previously never been expected.

[Another method has been devised by Landsbeiger {Her., 1898, 31, 458), and modified by Walker and Lumsden (./. Ckem. Soc., 1898, 78, 502).]

Advantages of the Freezing Point Method.— The determination of the boiling point or the vapour pressure does not permit of the calculation of the molecular weight of such dissolved substances as have themselves an appreciable vapour tension. The method of the freezing point is free from this disadvantage, for it is only the vapour pressure of the solvent which plays any imrt in it; thus, for instance.

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