368 REPORTS ON T'HE STATE OF SCIENCE.—1912. 
of a solid solution is too great to allow of equalisation of the solid solu- 
tion by diffusion in order to re-establish equilibrium. Annealing at a 
temperature below the solidus curve favours diffusion, and usually 
brings about equalisation in a short time, measurable in hours. 
The best zonal structures are obtained when the two end-members 
of the series composing the crystal have nearly identical molecular 
volumes, as in the triclinic felspars, in which the variation is only 
from albite, 100°18, to anorthite, 101°49.1*° 
The most favourable mstances for the view that reactions involving 
diffusion may take place within a crystalline mineral are those of 
schiller-inclusions, as of magnetite in the olivine from peridotite, Isle 
of Rum,**! and in the hypersthene from norite, Labrador, and also of 
rutile in certain felspars and pyroxenes. Such structures may well be 
due to the change in concentration of a solid solution during cooling 
causing the separation of the constituent in excess. The microperthitic 
intergrowths of albite and orthoclase in alkali-granite (Rockport, Mass.) 
and of albite and microcline in nepheline-syenite (Miask, Ural) are also 
instanced by Harker as being very possibly formed in the solid state. 
These structures are strongly suggestive of the eutectoids of certain 
metallic alloys. ‘ Pyroxene-perthite,’ an intergrowth of rhombic 
pyroxene and diopside, suggests a similar origin.?*? 
Agate and chalcedony are to be classed among solid colloids rather 
than among crystalline minerals, but the suggestion that certain 
brecciated and banded structures observed in them are not of primary 
origin, but have arisen through segregation in the solid state,*** should 
not be overlooked. 
Finally, in this connection, mention may be made of the ‘ pleo- 
chreic halos’ observed in certain minerals, notably biotite, and attri- 
buted to the a radiation from minute enclosures of radioactive material. 
Each halo is made up of several concentric shells, each of which 
represents the range of some of the types of a particle emitted.*** 
Halos in all stages of development have been observed. Artificial 
pleochroic halos have been obtained in glass.*° The origin of the halo 
is of course not ordinary diffusion, as the a particles are shot off at 
great speed from the centre, but the penetration of the crystalline 
material appears certain. 
Diffusion in Artificial Crystals. 
The process of diffusion in crystalline salts or organic substances 
has been little studied. From the fact that isomorphous crystals 
frequently resolve themselves into their components on cooling, and 
that other crystals unite under similar conditions, it is certain that 
diffusion must take place, but there is usually no record of direct 
M0 'T. V. Barker, Trans. Chem. Soc., 1906, 89, 1120. 
Ml A, Harker, ‘ The Natural History of Igneous Rocks,’ p. 257. 
2 J. H. L, Vogt, Tscherm. Min. Mitt., 1905 [ii.], 24, 537. 
ue ee Ruskin, Geol. Mag., 1867-1870, 4-7; reprinted in ‘Collected Works,’ vol. 26, 
pp. 3 
a ‘s Joly, Phil. Mag., 1907 [vi.], 18, 381; J. Joly and A. L. Fletcher, ibid., 1910 
[vi.], 19, 630. 
145 FB, Rutherford, Mem. Manchester Phil. Soc.. 1909, 54, v. 1. 
