A new and most valuable method of determining the molecular weights of
non-volatile as well as volatile substances has just been brought into
prominence by Prof. Victor Meyer (_Berichte_, 1888, No. 3). The method
itself was discovered by M. Raoult, and finally perfected by him in
1886, but up to the present has been but little utilized by chemists.
It will be remembered that Prof. Meyer has recently discovered two
isomeric series of derivatives of benzil, differing only in the
position of the various groups in space. If each couple of isomers
possess the same molecular weight, a certain modification of the new
Van't Hoff-Wislicenus theory as to the position of atoms in space is
rendered necessary; but if the two are polymers, one having a
molecular weight n times that of the other, then the theory in its
present form will still hold. Hence it was imperative to determine
without doubt the molecular weight of some two typical isomers. But
the compounds in question are not volatile, so that vapor density
determinations were out of the question. In this difficulty Prof.
Meyer has tested the discovery of M. Raoult upon a number of compounds
of known molecular weights, and found it perfectly reliable and easy
of application. The method depends upon the lowering of the
solidifying point of a solvent, such as water, benzine, or glacial
acetic acid, by the introduction of a given weight of the substance
whose molecular weight is to be determined. The amount by which the
solidifying point is lowered is connected with the molecular weight,
M, by the following extremely simple formula: M = T x (P / C); where C
represents the amount by which the point of congelation is lowered, P
the weight of anhydrous substance dissolved in 100 grammes of the
solvent, and T a constant for the same solvent readily determined from
volatile substances whose molecular weights are well known. On
applying this law to the case of two isomeric benzil derivatives, the
molecular weights were found, as expected, to be identical, and not
multiples; hence Prof. Meyer is perfectly justified in introducing the
necessary modification in the "position in space" theory. Now that
this generalization of Raoult is placed upon a secure basis, it takes
its well merited rank along with that of Dulong and Petit as a most
valuable means of checking molecular weights, especially in
determining which of two or more possible values expresses the
truth.--_Nature._
* * * * *
[Continued from SUPPLEMENT, No. 642, page 10258.]
THE DIRECT OPTICAL PROJECTION OF ELECTRO-DYNAMIC LINES OF FORCE AND
OTHER ELECTRO-DYNAMIC PHENOMENA.[1]
[Footnote 1: An expansion of two papers read before the A.A.A.S.
at the Ann Arbor meeting.]
By Prof. J.W. MOORE.