181 



THE GEOLOGIST. 



saltic rock in the serpentine rock in the Valley of St. Nicholas, in Pied- 

 mont, in lavas in Vesuvius and Iceland, in beds of brown iron ore at 

 Saalfeld, the Harz, and Styria. 



We now come to the mineral calcite, calcspar, or Iceland spar. This 

 calcite is pure carbonate of lime. So far as analyses tell us, some speci- 

 mens of Iceland spar are absolutely chemically pure. Sometimes they 

 contain about one-half per cent, of water, and they not unfrequently enclose 

 foreign matters, such as copper pyrites and sand. 



"Next, as to the formation of calcite. Calcspar, or crystallized carbonate 

 of lime, crystallizes in the rhombic system. We have seen that it can be 

 produced by means of water ; we will now consider its production through 

 the agency of fire only. We have all heard of the famous experiments of 

 Sir James Hall ; they were commenced in 1798, and the results were com- 

 municated to the Royal Society of Edinburgh. He informs us that he 

 took amorphous carbonate of lime, or chalk, and by exposing it to a high 

 temperature under considerable pressure, he succeeded in converting it into 

 saccharoidal limestone, like Carrara marble. The lecturer had had an op- 

 portunity of seeing one specimen prepared by Sir James Hall, and he must 

 say that the result did not strike him as conclusive. But now for the evi- 

 dence. He enclosed carbonate of lime in gun-barrels, and resorted to vari- 

 ous expedients of plugging those gun-barrels, such as plugs of soft metal, 

 and so forth. He then exposed a portion of the gun-barrel to a high tempe- 

 rature, taking care to arrange the tube horizontally ih such a manner that 

 the plug of soft metal should not be melted ; and he obtained a hard sub- 

 stance like limestone after having exposed chalk to these conditions. He 

 says, " My first application of this scheme was carried on with a common 

 gun-barrel cut off' at the touch-hole, and welded very strongly at the breech 

 by means of a plug of iron. Into it I introduced the carbonate, previously 

 rammed into a cartridge of paper or pasteboard, in order to protect it from 

 the iron, by which, in some former trials the subject of experiment had 

 been contaminated throughout during the action of heat. I then rammed 

 the rest of the barrel full of pounded clay, previously baked in a strong 

 heat ; and I had the muzzle closed like the breech, by a plug of iron welded 

 upon it in a common forge, the rest of the barrel being kept cold during 

 this operation by means of wet cloths." This gives you an idea of one of 

 his experiments. Then he comes to the use of fusible metal. He employed 

 tubes of glass. It is desirable particularly to examine the evidence upon this 

 subject, because it is one on which much stress has been laid. The lecturer 

 did not wish to question unnecessarily the accuracy of Sir James Hall's 

 conclusions, but he might remark that the carbonate of lime being heated to 

 a high temperature in contact with glass, the result would be altogether 

 vitiated, and the crystallization could not be said to depend merely upon 

 the outward conditions to which the substance was exposed. We find that 

 in other experiments he used snfall quantities of carbonate of lime in con- 

 tact with silica and clay ; but the presence of these two bodies would very 

 much modify the result. In other experiments he used borax, and that 

 again would altogether vitiate the result. Therefore, the conclusions 

 drawn from these experiments are unworthy of being received — at all 

 events, without further evidence. He tells us that in several cases the 

 material which he obtained, although resembling crystalline limestone, fell 

 to pieces on exposure to the air. That, however, is not the property of 

 crystalline limestone. !No doubt the investigations of Sir James Hall were 

 conducted with perfect honesty and candour, and they must have involved 

 a great deal of expense ; but, as far as we know, recourse was never had 



