Bartow—A Mechanical Cause of Homogeneity of Orystals. 655 
to the often great want of correspondence between the optical 
properties of two isomorphous crystals. 
The fact that the densities of isomorphous mixtures are 
weighted-means of the densities of their constituents militates 
against the supposition that there is any change in the relation of 
the parts of these constituents such as would alter properties; 
whatever change exists, the parts of a chemical molecule which 
enters into a mixture must be supposed to persist and perform 
the same functions as they do when unmixed.? 
Perhaps the evidence which most strongly supports the view 
that the dissimilar constituents in two isomorphous bodies occupy 
definite similar situations, being symmetrically and regularly in- 
terspersed each throughout the compound of which it is a consti- 
tuent in a similar manner, is afforded by the valuable researches 
of Tutton. 
This resourceful and original investigator has made a series 
of measurements of the isomorphous crystals of the monosym- 
metrical double sulphates of the composition R, SO,, R” SO,, 6H,0. 
Each of the 22 salts measured contains, as the metal R, one of’ 
the three alkali metals—potassium, rubidim, or caesium ; the salts 
may hence be arranged in series of three, containing the same 
dyad metal R”’, but different monad ones. The measurements 
show that all the geometrical properties of the rubidium salts are 
intermediate between the corresponding properties of the potas- 
sium and caesium salts respectively. ‘The same is also true of the 
facility of crystallization and the crystalline habit assumed by the 
rubidium salts. Since the axial angles of the rubidium salts are 
very approximately a mean between the corresponding ones of 
potassium and caesium, and the same relation approximately sub- 
sists between the atomic weights, the probability suggests itself 
that the differences observed in the properties are due to the sur- 
1 Comp. Tutton. Journ. Chem. Soc., 1894, p. 628. 
2 See Retgers on the specific weight of isomorphous mixtures. Zeitschr. f. phy- 
sikal. Chemie, vol. iii., p. 497. Similar relations subsist for other properties. Comp. 
Brauns’ ‘‘ Optische Anomalien der Krystalle,” p. 205. This author notes, too, that 
mixed crystals whose constituents when crystallized alone give, the one negative, the 
other positive crystals, can contain such proportions of the two constituents as to be 
isotropic. J0., p. 237. 
SCIEN. PROC. R.D.S., VOL. VIII., PART VI. 3B 
