166 Dr. Thomson on Coal Gas. 



A ton of Lesniahagow coal, when distilled at the usual tem- 

 perature, yields about (10,080 cubic feet) one-fifth of its weight 

 of gas, two-fifths of coke, and two- fifths of tar, water, &c. 



The gas contains about one-fifth of the carbon in the raw 

 coal, two-elevenths of the hydrogen, and two-ninths of the 

 oxygen ; about one-half of the carbon remains in the state of 

 coke, so that about two-fifths go to the formation of the naph- 

 thalin, naphtha, naphthene, naphthol, &c., which are formed 

 during the distillation. 



Nine-elevenths of the hydrogen and seven-ninths of the 

 oxygen go to the formation of water and various other com- 

 pounds. The ammonia formed amounts to about 1 per cent, 

 of the liquor obtained during the distillation of the coal. 



When gas-works were first established the coal was distilled 

 in iron retorts, but it has been found more oeconomical to sub- 

 stitute vessels of stoneware, or rather indeed ovens of fire- 

 brick made air-tight; these I believe are generally superseding 

 the iron retorts. 



During the course of last winter I made thirty-five analyses 

 of gas from different gas-works, but most commonly Glasgow 

 gas. The gas which I used was taken from a pipe at some 

 distance from the gas-works, because the gas required to be 

 washed and purified before it was examined. After turning the 

 stopcock the gas was kindled and allowed to burn for several 

 minutes before I began to collect it. In every case it con- 

 tained a mixture of common air, which varied in different spe- 

 cimens of gas from 4 per cent, to 28 per cent. ; the mean quan- 

 tity was 12^ per cent. The specimen containing 28 per cent, 

 of common air was brought up from Greenock, and though 

 very great care was taken in packing it, it is possible that at 

 least a portion of this air might have made its way into the bot- 

 tles during the transit. If we omit this specimen, the average 

 quantity of common air in the Greenock gas was 10^ per cent ; 

 the average quantity in the Glasgow gas was 1 2i per cent. 



1 think it most likely that the common air, which forms a 

 constant ingredient in all gas from gas-works that I have ex- 

 amined, had made its way into the pipes, which it must be 

 very difficult to make air-tight ; and when the pressure is re- 

 moved common air will undoubtedly enter wherever it can 

 find access. The Greenock gas was collected in an apart- 

 ment very near the gas-works ; the Glasgow gas was collected 



was found to be covered over with a fine smooth, shining black coat of 

 carbon, which had been deposited on it." Mr. Hart has conckided from 

 this and other facts, that the gas is decomposed by the red-hot coke, and 

 proposes that the coal should be applied in thin layers to the interior sur- 

 face of the retorts. 



