184 
THE GEOLOGIST. 
saltic rock in tlie serpentine rock in tlie Valley of St, Nicliolas, 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 aud sand. 
In ext, 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 Eo3'al 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 in 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 ofl' 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 ^vhich, 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 small 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 
