268 Dr. Mac Culloch on the Vitrifed Forts of Scotland, 



integrity, as it is known to be low. It is equally useless to ex- 

 amine 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 amygda- 

 loid was sometimes unaltered, and sometimes disintegrated by the 

 calcination of its lime, without undergoing any mark of fusion 

 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 j 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 unnessary to relate all the experiments which I per- 

 formed on the diflferent substances, as it is not my object to state 

 these matters for the purposes of chemistry. Those which have been 



