Mr. F. C. Calvert on the Adultei-ation of Oils. 101 



kaolins, which are merely decomposed granitic rocks, undergo in 

 the alkali losses much greater than those of granitic rocks. 



Other conditions being the same, the action of alkalies on 

 rocks increases with the amount of silica, with the absence of 

 crystalline structure and of hyaline quartz. On this account, 

 vitreous rocks which contain little or no quartz, as retinite, obsi- 

 dian, trachyte, are veiy strongly affected by alkalies. 



When instead of alkalies we use alkaline carbonates, some 

 rocks, especially the \dtreous, are still acted upon, but much less 

 energetically than by the caustic alkalies. 



The readiness with which alkalies, and even alkaline car- 

 bonates, act upon rocks, shows how cautiously they should be 

 used for the separation of the free and directly soluble silica that 

 may be mechanically mixed in a rock, as particularly in the 

 kaolins and the finer clays. 



I may remark, that, for instance, in obsidian the silica dis- 

 solved is not free, but in the state of soluble silicate; more- 

 over, in I'etinite the silica is not in the state of opal, as is sup- 

 posed, but in that of soluble hydrosilicate. Generally in all 

 vitreous or porphyi-itic rocks, hydrous or anhydrous, the silica is 

 held in a combination, not definite, forming the paste of these 

 rocks, and which is attacked by the alkali. 



The water penetrating rocks by infiltration always contains, 

 even near the surface of the earth, small quantities of alkaline 

 salts ; it is easy, then, to conceive that these salts should 

 aid in decomposing rocks and in producing pseudomorphisms. 

 But at a greater depth water becomes largely impregnated with 

 alkaline salts, the temperature and pressure increasing rapidly ; 

 it then influences greatly the rocks with which it is in contact. 

 This is the case with the water of minei*al springs, of the geysers, 

 of mud volcanos, and in general of volcanic regions. Conse- 

 quently the action of the alkalies and of the alkaline salts plays 

 an important part, not only in the formation of pseudomorphs, 

 but also in the chemical reactions which take place in the interior 

 of our planet. 



XV. Oh the Adulteration of Oils. 

 By F. Crace Calvert, Esq."^ 



IN consequence of the large quantities of oils used at the pre- 

 sent day for machinery, woollen, &c., many varieties are 

 introduced into the market, and mucli temptation exist.s to mix 

 or adulterate the most expensive ones. Having been at various 

 times called upon to examine sami)les of oil, I ascertained that 

 the processes known to discover adulteration were too general in 



* Cominumcated by the Author. 



