which possesses the characteristic Properties of Tannin . 143 
Every circumstance seems therefore to support the opinion 
of those who consider the pit-coals as having been formed in 
the humid way, principally from vegetable bodies, and most 
probably by the agency of sulphuric acid ; and allowing that 
animal substances may also have contributed to the production 
of coal, yet this would not militate against the above men- 
tioned opinion, as the effects produced upon them by that 
acid would in all the essential points be perfectly similar.* 
An inquiry into the nature and formation of coal was my 
first object when I discovered the artificial tanning substance, 
take place ; for the intimate combination of the alkali with the woody fibre becomes 
in a great measure destroyed by the carbonization of the latter, whilst a simultaneous 
action arises in the affinity between the acid and the alkali ; so that if coal has been 
formed by such means, the alkali must have been separated from the wood in the 
state of a dissolved neutral salt. 
* From the nature of the experiments which have been related in this Paper, I 
have unavoidably been induced to notice concisely the different opinions on the 
formation of coal by the humid way ; but I did not intend to have mentioned any of 
those which have been brought forward in favour of the immediate or indirect action 
of fire, as I only wished to express my sentiments respecting the most probable of the 
former opinions. 
Since however this Paper was written and partly read before the Royal Society, I 
have been favoured by Sir James Hall, with a copy of his Paper, intitled 
“ Account of a Series of Experiments shewing the Effects of Compression hi modi- 
-'‘lying the Action of Heat and I am fully of opinion that the scientific world 
has not for a long time received any communication of more importance, or 
in which more accuracy, ability, and perseverance have been displayed. The 
effects which Sir James Hall has produced on carbonate of lime by heat acting 
under compression, certainly removes a great and at one time apparently insur- 
mountable obstacle to the Huttonian or Plutonian theory, and if they do 
