168 



Professor Thomas Thomson on Cod Gas. 



a substitute for caudles. These attempts led to no results, and were 

 speedily forgotten. 



There are four varieties of coal which have been tried in Great 

 Britain in the manufacture of gas ; namely, caking coal, cherry coal, 

 splint coal, and cannel coal. Of these, the cannel coal or parrot coal, 

 as it is called here, yields the best gas ; the caking coal or Newcastle 

 coal yields the worst ; and the cherry and splint, though very different 

 in their appearance, yield an intermediate gas, the quality of which, 

 whether from cherry or splint coal, is nearly the same. 



There are three varieties of cannel coal in the neighbourhood of 

 Glasgow, named from the localities where they occur, Skaterig, Les- 

 mahagow, and Monkland. 



The specific gravity of these varieties of coal is as follow : — 



Besides ashes, these six varieties of coal consist of carbon, hydrogen, 

 azote, and oxygen combined in various proportions according to the 

 coal. I shall here give the composition of each. That of the three 

 first was determined by Mr. Richardson of Newcastle, in the labora- 

 tory of Giessen ; that of the three last in my laboratory, by my 

 nephew, Dr. R. D. Thomson. The azote is small in quantity — so 

 small, that Mr. Richardson did not succeed in determining its exact 

 quantity ; but we found no difficulty in coming to pretty exact con- 

 clusions by the process of Will and Varrentrap. As the quantity in 

 all our varieties tried varied from 1-48 to 1*75 per cent. I have sup- 

 posed that the azote in the three varieties determined by Richardson 

 was the mean of these two quantities, or 1*61 per cent. The following 

 table shows the composition of these coals ; — 



It will facilitate our conception of the composition of these different 

 coals, if we exhibit their constitution by empyrical formulas, repre- 



The specimen was marked Knightswood. 



