IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 331 
artificially, or liquefied in its combination with potash, becomes 
inactive, even when care is taken to form the combination with 
portions of the same crystal acting in one and the same direction, 
as Sir John Herschel has done. Quartz résinite, opal, tabasheer, 
opaline quartz of Mexico, some specimens of which present 
the most perfect limpidity, are also devoid of rotatory power, 
although these substances are composed of silicic acid, like rock 
crystal, from which they only differ by the absence of crystal- 
lization, and probably by a greater or less amount of combined 
water. To reconcile these facts with the supposition that the 
double power exercised by crystallized quartz was molecular, we 
must admit first that the silicic acid exists in it in two different 
states, of which we have no proof; for that would be necessary 
to explain the occasional alternation of the deviations toward the 
right or left of the observer. It would moreover be necessary 
that artificial fusion, as well as all the natural causes which de- 
stroy crystallization, should also destroy this dissimilarity, and 
reduce the acid to an inactive state, without its ever again ac- 
quiring its power in any of the combinations into which it might 
be made to enter, which would form a single exception to all the 
analogous cases. It is far more probable that the rotatory pro- 
perties of the crystallized quartz result from the mode of super- 
position of the crystalline laminze composing the crystals, which, 
when broken transversely, always present an helicoidal struc- 
ture, arranged around the longitudinal axis passing by each of 
their internal points. This would explain why the optical effects 
thus produced are especially regular in the central portions of 
the plates, situated near the principal axis of the crystals; whilst 
towards the outer edges, where the continuity of the crystalli- 
zation has been modified by the lateral causes limiting it, they 
generally appear irregular and of variable intensities, even be- 
coming sometimes completely evanescent. We might thus again 
conceive how crystals possessing contrary rotations may have 
been agglutinated laterally, in a simultaneous formation, so as to 
furnish transverse plates presenting this opposition in the differ- 
ent portions of their surface, with alternations sometimes in- 
finitely multiplied, as in the variety of quartz called amethyst ; 
whilst so varied a dissimilarity of molecular state would be in- 
comprehensible in such a simultaneity and proximity of deve- 
lopment. Lastly, what confirms the foregoing inductions is, 
that according to a remarkable observation due to Sw John 
