C. H. Warren- — Calcium Carbide. 127 



Reaction with water. — The effect of slow decomposi- 

 tion of the carbide by water, as seen under the micro- 

 scope, is curious and worthy of note. This effect is best 

 observed if a few thin rectan^-ular cleavage fragments 

 are inimersed in glycerine containiui>' a very little water so 

 as to cause the decomposition of the carbide into acetyl- 

 ene and calcium hydroxide to proceed very slowly and 

 without effervescence enough to disturb the position of 

 the grains. Viewed between crossed-nicols the margin 

 of the grains may be seen to gradually lose the strong 

 interference colors due to the birefringence of the car- 

 bide, and a rim of feebly birefringent material begins 

 to develop. The corners naturally change more rapidly 

 than the sides so that the unchanged carbide within 

 assumes a circular or elliptical outline, which constantly 

 diminishes in size until the entire grain has changed to 

 the feebly doubly-refracting (about 0.008) material, which 

 is calcium-hydroxide. During this change the structural 

 appearance of the grain does not alter, cleavage cracks 

 and twinning lines remaining. After a lapse of a little 

 longer time the substance breaks up, separating along 

 the directions of the cleavages, and finally dissolves in the 

 liquid if there is sufficient water present. The change 

 appears to involve the passage of one crystalline sub- 

 stance to another without the immediate breaking down 

 of the essential crystal structure. 



In the reaction given by the equation 



Caa + 2H0H =: Ca(0H)2 + aH^ 



we might suppose that perhaps the OH group simply takes 

 the place of carbon atoms, the crystal structure being pre- 

 served intact by the network of calcium atoms, or at least 

 some such mechanism is conceivable. This phenomenon 

 recalls to mind the preservation of crystalline structure 

 in certain zeolites when slowly dehydrated. It also occurs 

 to the writer, that the change of biotite to chlorite, so 

 frequently seen in rocks, where the chlorite seems to sim- 

 ply replace the original biotite, the brown color and 

 strong birefringence of the latter changing to the green 

 color and low birefringence of the former, is also a sub- 

 stitution of certain atoms or atomic groups in the crystal 

 structure, the main features of which remain intact 

 throughout the change. 



An X-ra}^ study of calcium carbide and its decomposi- 



