CunnincHamM—Crystallization of Minerals in Igneous Rocks. 385 
fibres depend on rapid cooling, and are destroyed by annealing.! 
The physical properties of quartz are in many respects so peculiar 
and evidently so closely connected with a particular molecular 
arrangement,” that it is a peculiarly unsafe thing to infer its 
behaviour from experiments on amorphous silica which presents 
more normal characters. It would be as safe to infer the physical 
properties of the diamond from experiments on graphite or gas 
carbon. The refractive index of quartz for the D line is 1°5442, 
that of amorphous silica 1:4587 (according to a communication 
from Prof. C. V. Boys). The density of quartz is 2°66; that of 
amorphous silica 2°22. Hence, in the latter, the average distance 
of the molecules of SiO, may be supposed to be greater, and hence 
the intermolecular forces will be less; we should not then be sur- 
prised to find that amorphous silica is not so rigid under stress as 
quartz. It is well known that the physical properties of the useful 
metals are profoundly altered by their method of preparation and 
previous treatment, although the actual difference of molecular 
structure is probably much less in such cases than between quartz 
and amorphous silica. I may quote an example of this taken from 
the work of Dr. Carl Barus*® as being connected with the subject 
in hand in more ways than one. He plots curves exhibiting the 
results of experiments on hard and soft steel. It will be seen at a 
glance how much greater is the viscosity of soft steel than of hard 
steel, and how much more rapidly it increases with time in the 
former case than in the latter. In fact only at the end of a consi- 
derable interval of time does the absolute viscosity of hard steel 
attain to that of soft specimens at the outset of the experiment. 
At the same time we may note that there can be no doubt that 
the soft steel is actually the more ductile, 7.e. it can be stretched 
more easily without breaking. 
2. This naturally leads me toasecond point. Prof. C. V. Boys* 
1M. Gaudin, ‘‘Seconde lettre sur les propriétés du cristal de roche fondu.’” 
Comptes Rendus 8 (1839), p.711; and C. V. Boys, Proc. Phys. Soc. 9 (1887-88), p. 17. 
* Kelvin, ‘‘ On the Piezo-electric Property of Quartz,’’ Phil. Mag. [v.] 36 (1893), 
p- 331. 
3 C. Barus, ‘‘ The Change of the Order of Absolute Viscosity encountered on passing 
from Fluid to Solid,’’ Phil. Mag. [v.] xxix. (1890), p. 337. 
4C. V. Boys, ‘‘On the Production, Properties, and some suggested Uses of the 
Finest Threads,’’ Proc. Phys. Soc. 9 (1887-88), p. 17. 
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