CunnincHam—Crystallizsation of Minerals in Igneous Rocks. 391 
to this, he restates it rather more briefly :—‘“I defined a fluid 
(liquid or gas) as a body which, under constant conditions of 
pressure, temperature, and stress, shows constant viscosity as to 
time. In a solid, ceteris paribus, viscosity markedly increases 
with the time during which stress is brought to bear.’ An 
examination of the data given by Dr. Joly (loc. cit., p. 299) shows 
that his fibre was solid according to this definition. Thus during 
the first fifty-five minutes the extension was found to be 0:076cm., 
and during the next thirty-five minutes only 0:039, instead of 
0-048 ems., which it would have been had viscosity been constant. 
But, on the contrary, viscosity increased, as is quite characteristic 
of the flow of solids. 
4. Dr. Joly does not, indeed, state that the “melting point” 
of quartz is as low as 800°, but unless he intends to imply 
as much it is difficult to understand his meaning. Tor instance, 
he writes (/oc. cit., p. 3801) of the silica entering ‘as an influence 
retarding crystallization and prolonging the viscous properties 
downwards in the scale of temperature. CaO, MgO, Al,O;, on 
the other hand, are crystallizers at high temperature,” the contrast 
being apparently between these and silica, a ecrystallizer at low 
temperature. It certainly is the crystallizing temperature which 
concerns us in the present discussion. 
Joubert, in 1878, measured the rotatory power of quartz 
through a wide range of temperature, and found that, 
“De —20 a 1500 degrés, le pouvoir rotatoire du quartz augmente, 
d’une maniére continue, avec la température.” And further, 
using both right- and left-handed specimens, he found that, on 
allowing each specimen to cool down again, its rotatory power 
returned to its original amount. Now silica, which has once been 
really fused, has lost for ever this rotatory power. This question 
was fully discussed and put to experimental tests by Gaudin’ and 
1M. J. Joubert, ‘‘ Sur le pouvoir rotatoire du quartz et sa variation avec la tem- 
pérature.’? Comptes Rendus, 87 (1878), p. 497. The author marks the 1500° with 
a ?, but states that it was the softening point of porcelain. Frémy quotes the figures 
into his ‘‘Encyclopédie Chimique,’? and omits the?, evidently satisfied with the 
estimate. 
2M. Gaudin, ‘Sur les propriétés du cristal de roche fondu.’’ Comptes Rendus,. 
8 (1839), pp. 678, 711. 
