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XIV. On the Action of Alkalies on Rocks. J?y M. Delesse*. 



IN studying the action of alkalies on rocks, I treated the 

 powder of the specimen under examination in a solution 

 containing five times its weight of potash ; I then sought what 

 substances existed in the new solution. 



Since many rocks contain water, I dried and weighed the 

 residue from the potash solution, I then heated it to redness and 

 weighed again : I thus obtained for the different rocks com- 

 parable results, as shown in the following table : — 



Some general conclusions can be drawn from my researches. 



When an alkali attacks a rock, it dissolves from it not only 

 silica, but also alumina, water, potash and soda. Besides a little 

 lime, magnesia and traces of oxide of iron are taken up by the 

 alkaline liquor. 



The quantity of silica dissolved is always greater than that of 

 all the other substances. 



Granite may be said to be unaffected by boiling in a solution 

 of alkali ; quartziferous porphyry only loses a few hundredths. 



Lava, basalt and melaphyre lose not more than 20 per 100. 



Trachyte, retinite, perlite and obsidian are the most attack- 

 able ; within my experience their loss did not exceed 40 per 100. 



A rock containing water is much less easily affected by the 

 alkalies after it has been calcined. Thus, in the perlite from the 

 Cape of Gatis, the action of the alkali before and after calcining 

 varied in the ratio of 2|- to 1 . 



On the other hand, a rock is much more easily affected when 

 in a state of decomposition. Thus, argillaceous eurites and 



* Communicated by Su- Robert Kane. 



