281 
ON THE PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 
' BY HENRY LETHEBY, ESQ., M.E., ETC. 
Delivered at Birmingham before the Society of Gas Engineers. 
Gentlemen,—I propose that we should continue to-night our inquiries into the che- 
gas-lighting—that, in fact, we should extend our investigation into the subject 
which was commenced at the meetings of this Association last year at Manchester, when, 
as some of you will remember, I directed your attention to the chemical principles in¬ 
volved in the manufacture and purification of coal gas. On the present occasion we will 
examine the leading physical, chemical, and photometrical properties of the most impor¬ 
tant constituents of coal gas. 
Briefly to recapitulate the subjects of the last lecture, I may remind you that we in¬ 
quired, m the first place, into the composition and probable origin of the material out of 
which gas is produced coal; that we then examined the leading constituents of the se¬ 
veral varieties of coal which are best suited for the manufacture of gas, especially directing 
attention to the form in which the most objectionable impurity (sulphur) of coal existed. 
We also considered the phenomena of carbonization or distillation of coal, and saw how 
much it was influenced by temperature, and how, under the influence of heat, the ele¬ 
ments moved from their old states of combination into new. 
We then discussed the composition of raw gas as it leaves the retort,* and I pointed 
out to you that it consists of the constituents marked in the following table:— 
Constituents of Raio Gas • 
Tar matter. 
Aqueous vapour. 
Carbonic acid. 
Ammonia. 
Cyanogen. 
Sulpho-cyanogen. 
Sulphuretted hydrogen. 
Bisulphide of carbon. 
Sulpho-hydrocarbons. 
Nitrogen. 
Oxygen. 
Hydrogen. 
Light carburetted hydrogen. 
Carbonic oxide. 
Condensable hydrocarbons. 
Now the whole of these constituents above the line may be regarded as impurities, 
and I pointed out to you that the taking of these impurities out of coal gas, in order to 
effect the perfect removal of them, involved a proper order, as it were, of purification—■ 
that, in the first place, both science and practice pointed to the fact that the condensa¬ 
tion or cooling of the gas should not be too sudden; that the longer the gas was kept 
in contact with tar and ammoniacal liquor before it went to the condensers the better. 
It had been seen in numerous instances that a long hydraulic main, extending a consi¬ 
derable distance from the retorts, always effected the condensation of naphthaline as well 
as objectionable sulphur-compounds ; and I further pointed to the fact that when the 
gas has traversed the condensers it was never fairly purified if it left them at a higher 
temperature than 60° Fahr. The aim, in fact, should be slow but complete condensa¬ 
tion by gradual cooling, for if the temperature of the gas exceeded GO 0 the ammoniacal 
• liquor was never of its full strength, and much sulphur, ammonia, carbonic acid, and 
aqueous vapour passed on to the purifiers, where such impurities were seriously in the 
way. 
The next question is how to remove from the gas the sulphuretted hydrogen, the am¬ 
monia, and the other impurities which still remain in it. In continuation of the system 
already adverted to, it is well to submit the gas to the action of a copious stream *of am- 
YOL. vii. ' u 
