Dr. Thomson on Coal Gas. 165 



Caking .... C127H53AZO4 



Cherry .... CjaiH^^AzOg 



Splint C120 H45 Az Oio 



Skaterigg. . . C102H43AZO15 



Lesmahagow . Cuq Hgg Az O14 



Monkland . . CmHsgAzOig 

 It appears from this table that Newcastle coal contains the 

 most carbon, and Monkland cannel coal the least; while can- 

 nel coal contains the most oxygen and Newcastle coal the least; 

 Newcastle coal contains the least hydrogen and cannel coal 

 the most. Now cannel coal yields the best and Newcastle coal 

 the worst gas. This need excite no surprise; carbon not being 

 volatile, it is obvious that if coal contained nothing but carbon 

 it would yield no gas at all. Coal-gas is a mixture of four 

 different gases, most of which are compounds ; two are com- 

 pounds of carbon and hydrogen, one of carbon and oxygen, 

 and the fourth is pure hydrogen. There is no difficulty in con- 

 ceiving the formation of the gaseous compounds of carbon and 

 hydrogen, but it is not so easy to explain the occurrence of car- 

 bonic oxide and hydrogen. These two gases are never entirely 

 wanting; at least I have analysed above forty specimens of coal- 

 gas from different kinds of coal and from different gas-works 

 without ever failing to find them. I think it probable that 

 they make their appearance towards the end of the process of 

 heating the coal. It is well known that the longer the process 

 of gas making is continued, and the higher the temperature at 

 which the gas is produced, the worse is the gas, and of course 

 the more hydrogen it contains. Is it not possible that coal 

 may contain water ; that this water can only be extricated at 

 a high temperature ; that its oxygen combines with carbon and 

 forms carbonic oxide, while the hydrogen makes its escape in 

 the gaseous state? If this supposition were true, there ought 

 to be a constant ratio between the volume of carbonic oxide 

 and hydrogen in the coal-gas ; but this not being the case, it 

 is obvious that the supposition cannot be well founded*. 



* Mr. John Hart, of this city, states that he made an experiment which 

 appears to explain the appearance of the hydrogen towards the end of the 

 process. " Having conceived the idea many years ago of causing gas to 

 take up an additional dose of carbon, by passing it over red-hot charcoal, 

 he procured a IJ-inch iron pipe, and having charged it with charcoal he 

 passed it through the furnace below the gas retort, and joined one end 

 with the pipe from the retort and the other to the pipe leading to the con- 

 denser; the fire was then applied and the retort charged as usual. After 

 the gas-holder had risen about a foot, he observed the lead pipe leading to 

 the condenser becoming very hot ; it soon after gave way and fell to pieces, 

 and the whole of the gas escaped into the air; but it had no longer the 

 yellow silky appearance of gas issuing from a retort; it had become a white 

 vapour, and had also lost the smell. When the charcoal was examined it 



