394 Prof. V. B. Lewes. [Mar. 7, 



primary or secondary solution planes, hydration of the aluminium 

 oxide takes place, and diaspore is formed, as shown by Lawrence 

 Smith and Genth, and this unstable mineral enters into combination 

 with silica and other oxides present to give rise to the numerous 

 pseudomorphs of corundum, which are so well known to mineralo- 

 gists. 



There are certain crystals of corundum and spinel from Burma, 

 which present illustrations of corrosion of a very remarkable and 

 interesting character. Commencing with the formation of naturally 

 etched figures (" Verwitterungsfiguren ") the work of corrosion goes 

 on till the whole crystal is broken up into an aggregate of simple 

 forms these being, in the case of the spinel, the octahedron, and 

 in the case of the corundum, a combination of the rhombohedron, 

 basal plane, and prism. 



It is interesting to note that the quartz, felspars, and other minerals 

 associated with the rubies and spinels of Burma, exhibit phenomena 

 of external etching and internal chemical change similar to those we 

 have been describing in the case of the gems. The study of the 

 whole of the phenomena throws much new light on the remarkable 

 changes which take place, at great depth in the earth's crust, in 

 minerals which, at the surface, appear to be of a very stable 

 character. 



II. " The Action of Heat upon Ethylene, II." By VIVIAN B. 

 LEWES, Professor of Chemistry at the Royal Naval 

 College, Greenwich. Communicated by Professor T. E. 

 THORPE, F.R.S. Received January 10, 1895. 



In a paper communicated to this Society in the spring of 1894,* I 

 showed that ethylene, when subjected to heat, was converted into 

 acetylene and methane, according to the equation 



and that the acetylene so formed either at once polymerised, forming 

 a large number of secondary products, or else decomposed to carbon 

 and hydrogen, according to the temperature at which the action was 

 being carried on. 



The fact that ethylene is one of the principal products in many 

 cases of destructive distillation, renders a knowledge of the conditions 

 affecting these changes of considerable importance, and the experi- 

 ments described in this paper were made with the view of ascertaining 

 the effect of rate of flow, area of heated surface, and dilution upon 

 the changes taking place. 



* ' Boy. Soc. Proc./ vol. 55, p. 90. 



