Page:Outlines of Physical Chemistry - 1899.djvu/35

 weight are also those which most readily form comparatively light and consequently volatile molecules, and it is par- ticularly to them that this method of determining the atomic weight is applicable.

In this manner the atomic weights of the following elements have been derived : Be, B, C, N, 0, F, Al, Si, P, S, CI, Ti, V, Cr, Zn, Ga, Ge, As, Se, Br, Zr, Nb, Mo, Cd, In, Sn, Sb, Te, I, Ta, W, Os, Hg, Tl, Pb, Bi. (In L. Meyer's 1 Modern Theories of Chemistry ' (English translation by Bedson and Williams) a list of the principal compounds studied is given.)

Bemar k— Comparing the atomic weights with the molecular weights (experimental) we see that the molecule of an element often consists of 2 atoms, sometimes of only 1 atom, and occasionally of 4 or even 8 atoms. '

Thus we know P 4, As 4 , S 2 and S 8 , Cd and Hg.

Abnormal Vapour Densities

1. Small deviations. — Gases do not exactly obey the laws expressed by the general equation ; for moderate pressures, however, the deviation is only small.

2. Too high densities. — For certain substances the den- sity is too high when determined too near the boiling point.

For instance :

Acetic acid at 250°, Density = 2 # 08 (normal value). „ „ „ 125° „ =8-20.

8. Densities considerably too low are sometimes found, which lead to a molecular weight smaller than that which corresponds to the lowest possible formula (containing no fractions of atoms).

For instance, ammonium chloride. Analysis leads to the formula N n H 4n Cl n, and the smallest value of this is

rq.qQ

and a vapour density = 1*85. But Bineau has found

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