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XXII. On the Purification of Coal Gas. 
To Mr. Tilloch. 
Sir, — ‘Tux methods recommended by Messrs. Parker and 
Lowe, in late Numbers*of your Magazine, for the purification of 
coal gas, if they do not fully effect the object proposed, entitle 
those gentlemen to every praise by making the subject a matter 
of discussion and inguiry. 
It has always appeared to me, that when the various products 
derived from the destructive distillation of coal are made to pass 
through an iron tube heated to ignition, the carburetted hydrogen 
so obtained principally proceeds from the tar; and that that por- 
tion of carburetted hydrogen, which had been previously formed 
from the coal, undergoes another arrangement of its elements by 
traversing the heated medium. If, for example, coal gas be 
passed through a red-hot tube a/ter the tar has been separated, 
it will be found that the brilliancy of the light from gas so 
treated falls far short of what it would be, had it received no 
such treatment. 
This being the case (as is shown by experiments) proves clearly 
the process referred to to be faulty and injurious. I have, sir, been 
induced to offer these remarks, from recent opportunities I have 
had, whilst travelling through the country, of visiting several gas 
establishments, and of witnessing the truly excellent effects of 
lime when judiciously applied. There are various modes in 
which lime is employed in the purification of coal gas, and the 
effect produced is in proportion to its causticity, its quantity, and 
the extent of its surface. 
If the lime be not in a state of causticity or thereabouts, it is 
certain a larger quantity must be used than is absolutely necessary 5 
and whatever proportion be employed, it is important that as 
much surface as possible be exposed in the purifying process. 
In no lime vessel that I have had an opportunity of seeing, 
are these advantages so completely combined as they are in the 
lime-machine of Mr. Clegg. At the Gas-Works lately erected 
in the city of Chester, there is one of this gentleman’s purifiers ; 
and by the obliging assistance of Mr. Leete (a very intelligent 
young’ man and superintendant of the works) I was enabled to 
make some experiments on the gas purified by this machine. 
Having at other places submitted the gas proceeding directly from 
the purifier to the action of superacetate of lead dissolved in wa- 
ter, I uniformly obtained a result which indicated the presence 
of sulphuretted hydrogen, by the test being instantly blackened ; 
but on repeating the experiment at Chester, I could not diseover 
any trace whatever of this combination, Mr. Leete then sug- 
gested, 
