258 Dr Davy's Miscellaneous Observations , 



examination of these, even with the aid of acids and the mi- 

 croscope, it is sometimes difficult to determine whether they 

 contain silica or not. In such instances, the aid above men- 

 tioned may be had recourse to with advantage. If a little of 

 the clay or sandstone, in fine powder, as abraded by a knife, 

 be placed on a slip of glass, and gently touched — slightly 

 rubbed with a rod of glass, its end somewhat rounded and 

 perfectly smooth — a peculiar harsh sensation will be im- 

 parted to the fingers holding the rod, very characteristic, and, 

 after a little experience, not to be mistaken. This test of 

 silica, in a state of fine division, or of minerals as hard, in a 

 finely divided state, may be useful to the inquiring traveller, 

 a slip of glass and a small glass rod being, with water, all 

 that is required. Provided thus, the geologist may in an 

 instant determine, with tolerable accuracy, whether such 

 finely-divided silica or hard mineral enter into the composi- 

 tion of the matter examined, even though in the form of de- 

 licate silicified infusoria, microscopic objects, such as occur 

 in chalk in some situations, and which, to be distinctly seen, 

 require to be exposed to a high magnifying power. 



4. Tin, when precipitated from a saline solution by means 

 of a carbonated alkali in the state of carbonate of lime, ad- 

 heres in part to the sides of the glass vessel in which the 

 precipitation is made. If minutely observed, it will often be 

 found that the precipitate adheres more firmly in some places 

 than in others ; and if a slip of glass be immersed when the 

 precipitation is taking place, it will be found that one of its 

 sides is more coated than the other, and that on the side on 

 which there is most precipitate, there it adheres much more 

 firmly than to the others ; from one side the carbonate is 

 easily removed, from the other with difficulty. On both sides 

 the precipitate, when examined by the microscope, is found 

 in the form of crystals, and most generally rhomboidal. 



Do not the facts mentioned tend to shew, that in this in- 

 stance an influence is exercised analogous to that of electri- 

 cal polarity? And, is not the property thus displayed of 

 carbonate of lime so adhering to glass, that quality on which 

 its efficacy as a cement depends ? Precipitated from sea- 

 water by the separation of the carbonic acid by which it was 



