114 On the Geology of South Wales. [Fex. 
opinion of others better qualified than myself. All I can offer is 
the supposition that the very abundant volatile (dry looking and dry 
feeling) vapour evolved (very pungent both to the eyes and nostrils) 
rapidly carrying off the caloric ina latent state, intervenes between 
the tar and the skin, and prevents the more rapid communication ; 
and that when the hand is withdrawn, and the hot tar adhering, the 
rapidity with which this vapour is evolved from the surface exposed 
to the air cools it immediately. 
While I was conversing in the place, I heard it said that if a 
man put his hand into the cauldron with a glove on, he would be 
dreadfully burnt ; but no one could say he had seen it tried. Not 
choosing to sacrifice a pair of gloves to the trial of an effect I had 
no belief in, I wrapped a newspaper double about my hand, and 
plunged it in up to the wrist. 1 retained my hand in the tar longer 
than I could when naked without feeling any pain. 
Iam, dear Sir, very truly yours, 
Twickenham, Nov. 1, 1816. Ricuarp DavyEnrorr. 
P.S. As the sense of feeling must differ much in different per~ 
sons, I have since tried to compare the sensation produced by hot 
water with that which I have described above; and I can say de- 
cidedly that I cannot bear the heat of water at 140° so long as that - 
of tar at 220°, 
Articte VI, 
Some Observations respecting the Geology of South Wales, 
By W. H. Gilby, M.D. 
Tue short, though excellent, paper of Mr. Martin, in the Phi- 
losophical Transactions for 1806, contains most of the particulars 
of any interest regarding the South Wales coal tract, and forms a 
striking contrast with the long and wearisome narration with which 
the pages of some of our publications are overspread. The follow- 
ing facts, however, which occurred to me in a late tour through 
South Wales, shortened and continually interrupted by heavy rains, 
appear to me new, and in some measure interesting :— 
1. Mr. Martin states, and so does Mr. Townsend, that the coal 
deposite of South Wales rests every where upon the mountain lime- 
stone, the course of which will be better understood by referring to 
the map annexed to Mr. Martin’s paper. Now the term mountain 
lime-stone has always been applied to a rock varying a good deal in 
colour, but pretty uniform as to its general character, and composed 
chiefly of carbonate of lime, with some properties of argillaceous 
matter. But in two places [ found a rock immediately subjacent to 
the coal strata, and heretofore considered as ‘part of the common 
mountain lime-stone chain, which is a variety of magnesian lime- 
2 
