Professor Thomas Thomson <m Coal Gas. 175 



inch cast-irou pipe, and having charged it with charcoal, I passed it 

 through the furnace below the retort, and joined one end with the 

 pipe from the retort, and the other end to the pipe leading to the con- 

 denser, the fire was then applied, and the retort cliarged as usual. 

 After the gas-holder had risen about a foot, we observed the pipe 

 leading to the condenser (which was of lead) becoming very hot ; it 

 soon after gave way and fell to pieces, and the whole of the gas 

 escaped into the air, but it bad no longer the yellow silky appearance 

 of gas issuing from a retort, it had become a white vapour, and had 

 also lost the smell. As we could not collect any more of the gas, we 

 withdrew the fire, and allowed the whole to cool down. When I took 

 out the charcoal to examine it, in place of its being acted upon by the 

 gas as I supposed, I found it covered all over with a beautiful smooth 

 shining black coat of carbon which had been deposited upon it. This 

 was extremely brittle, and started off like scales of iodine when 

 pressed upon by the nails. As the gas was mixed with part of a 

 former charge, we could not ascertain its quality, but it certainly did 

 not seem at all improved; indeed, the gas seemed rather to have 

 parted with a portion of its carbon, (by passing through the red hot 

 charcoal,) than to have acquired an additional dose. On this account 

 I did not prosecute the experiment farther. However, this deposition 

 of carbon in the solid form upon the charcoal led me to examine more 

 minutely the appearances of similar shining depositions upon pieces 

 of common coke, and also the deposits of carbon that were formed 

 both within and below the retorts of the Glasgow gas work. The 

 change, too, that was produced upon the flame of a piece of coal in an 

 open fire arrested my attention also. I observed that when it passes 

 up through a mass of glowing cinders, it loses its brightness, and 

 becomes of a dusky yellowish red colour like common hydrogen, just 

 as if the carbon had been abstracted from the hydrogen in its passage 

 through the cinders ; and, therefore, when I saw the report drawn up 

 by Dr. Henry, of the analysis of the gas obtained by him during dif- 

 ferent periods of the charge from the same retort, this deterioration of 

 the gas in the latter period of the charge appeared to me to proceed 

 rather from the deposition of the carbon held in solution by the gas in 

 coming into contact with the new formed coke in the exterior part of 

 the charge, than from an inferior gas being given off from the coal in 

 the centre of the retort, any more than from that part of the coal 

 which was in contact with the retort itself. And, therefore, to obtain 

 gas of nearly as uniform a quality as possible from the whole of the 

 charge, a change ought to be made upon the present form of retorts, 

 so as not only to apply the coal in a thin layer to the surface of the 

 retort, (as had already been pointed out by Mr. Maben of Perth,) but 

 also to protect the gas from the action of the incandescent carbon 

 when formed. 



