PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 287 
tlialin is absorbed by chlorine and also by bromine, the products being liquids which 
have considerable analogy to Dutch liquid. It is also absorbed by sulphuric acid. 
Naphthalin is not easily decomposed by heat; it is quickly deposited by cold, forming 
brilliant white scales, which have a tendency to collect in the bends of pipes, and 
wherever there is an impediment to the easy flow of the gas. These scales are slowly 
volatile at ordinary temperatures; they melt at a heat of 174° Fahr., and they boil at 
428°. 
And now, in reviewing the facts which have been brought before us in an examina¬ 
tion of the several hydrocarbons found in coal gas, the conclusions are— 
1. That they belong to different groups, or series of compounds, in which the pro¬ 
portion of carbon and hydrogen rise by successive increments of 2 + 2, and that the 
amount of hydrogen in them progressively decreases. 
In the Marsh gas series, for example, the proportion of hydrogen is always two more 
than the carbon, thus:—Marsh gas, or Methyl-hydride, CoHj; Ethyl-hydride, C 4 H G ; 
Propyl-hydride, C G II 8 ; Butyl-hydride, C s H 10 ; Amyl-hydride, C 10 H 12 , etc., all of which 
are found in the petroleums. 
In the next series, the alcohol radicals, which may or may not be present in coal gas, 
the proportion of hydrogen is only one more than the carbon, thus:—Methyl, C 2 H 3 ; 
Ethyl, C 4 H 5 ; Propyl, C G H 7 ; Butyl, C 8 H 9 ; Amyl, C 10 H n , etc. 
In the third series, which are the chief constituents of coal gas, the Olefiant gas series, 
the proportions of carbon and hydrogen are equal, thus:—Methylene, C 2 H 2 ; Ethylene, 
C 4 H 4 ; Propylene, C G H G ; Butylene, C S H S ; Amvlene, C 10 II 10 ; Capryolene, C 12 H 12 , etc. 
In the next compound, Acetylene C 4 H 2 , it is two proportions less; and in the Benzol 
series it is six proportions less, thus:—Benzol, C 12 II G ; Toluol, C 14 H 8 ; Xylol, Ci G H 10 ; 
Cumol, C 18 H 12 ; Cymol, C 20 H 12 , ete. 
And, lastly, - in the Naphthalin series it is at least twelve proportions less. 
2. We notice that the illuminating power of these hydrocarbons rises in proportion to 
the amount of carbon contained in a given volume of them,—marsh gas being the weakest 
and naphthalin the strongest. 
3. We perceive that their weight, or specific gravity, rises with their photometrical 
value,—marsh gas being only about half as heavy as atmospheric air (05531), and 
naphthalin four and a half times (4-42); so that, if accidental impurities were not pre¬ 
sent in coal gas, the specific gravity of it would furnish a good indication of its quality. 
4. We remark that all the richer hydrocarbons, excepting acetylene and naphthalin, 
are easily decomposed by heat, carbon being deposited, and a weaker quality of gas—- 
generally marsh gas—produced. It is manifest, therefore, that these important con¬ 
stituents of coal gas will not bear contact with the red-hot walls of the retorts; and the 
practical conclusion from it is that they should be swept out of the retorts as quickly as 
possible, and that the temperature of the retorts, especially of the upper parts, should be 
as low as possible; in fact, the destruction of these bodies is not by the temperature to 
which the coals are subjected, but by the heat of the upper parts of the retorts upon 
which the distilled gases and vapours impinge. 
5. We have observed that, with one exception (marsh gas), all these hydrocarbons 
are freely absorbed by chlorine, bromine, and strong sulphuric acid; and that in each 
case a very similar set of compounds is formed, Dutch liquor being the homologue of the 
haloid compounds. This indicates the difficulty of determining the photometrical value 
of gas by the amount of condensation with chlorine or bromine; for by such a process we 
Have no knowledge of the particular hydrocarbon condensed. In my own experiments 
I find that the condensation may be very different, even when the gases examined have 
the same illuminating power; and, conversely, we may have the same amount of con¬ 
densation for gases of very different illuminating powers: the number, therefore, 3 - 25, 
which is sometimes taken as the coefficient of power, is altogether unreliable. 
6. We notice that all the richer and more condensable hydrocarbons are condensable 
by cold, and, therefore, that gas should not be subjected to a cold of 32°, or even 
much below 50° Fahr. 
7. It is a fact, that water has little or no influence on any of the hydrocarbons, except 
acetylene; and as this exists in gas only to a very small extent, there is little or no 
danger from a copious washing of the gas before it goes to the purifiers. 
8. It is worthy of remark that the hydrocarbons are freely absorbed by oils, and by 
vulcanized india-rubber tubing. This circumstance should be taken into account in 
