Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/278

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Dr. Mac Culloch on the Vitrified Forts of Scotland.

integrity, as it is known to be low. It is equally useless to examine into the powers of the granite and quartzy rocks to resist heat, as they also are well known.

The fusibility of the pudding-stone arises partly from the fusible nature of the substance which I have described as forming its proper paste, but in some measure also from that of the amygdaloids and greenstones imbedded in it. It is in consequence of the carbonic acid contained in the calcareous crystals which these amygdaloid pebbles exhibit, that the inflated scoria are produced; for it may be easily traced to them through a regular gradation. To pursue this subject experimentally, I thought it necessary to submit various parts of this rock to the furnace, that I might ascertain the degree of heat necessary to effect the corresponding changes in it, and the fragments were accompanied by one of Mr. Wedgwood's pyrometer pieces.

The spongy scoria remained unchanged, and the natural amygdaloid was sometimes unaltered, and sometimes disintegrated by the calcination of its lime, without undergoing any mark of fission in a heat of 20°; a heat at which brass is melted. From 20° to 30° the amygdaloid underwent no change except a slight vitrification on the surface; at 40° it was much affected, and was fused into a glass at 60°.

Having excited the fire to 100° I exposed to it various parts of the pudding-stone, which had not been affected in the heat at which the amygdaloid was changed. Some of these were vitrified, and became precisely similar to many of the specimens taken from the wall, whilst others continued to resist, for a long time, even this intense heat; a heat at which many varieties of earthenware are baked. It is unnecessary to relate all the experiments which I performed on the different substances, as it is not my object to state these matters for the purposes of chemistry. Those which have been