286 PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 
feet of gas. The vapour of benzole is colourless, and it has a peculiar odour, which is 
rather agreeable when the benzole is pure. It is powerfully anaesthetic, and will cause 
fatal insensibility if it be inhaled too copiously. The vapour is about 2f times as heavy 
as atmospheric air, its specific gravity being 2-695 ; 100 cubic inches of it will, there¬ 
fore, weigh 84 specific grains. The vapour burns with a very bright light—so much so 
that I have no difficulty in giving a strong illuminating power to hydrogen by merely 
passing the gas through a tube containing a little tow moistened with benzole. This is 
a good example of the naphthaiization of gas; and I have here another instance of it 
where coal gas is passing over the benzole contained in a proper naphthalizer. You per¬ 
ceive the extraordinary richness of the light. Experiments have been made for the pur¬ 
pose of determining the value of the light for the benzole or naphtha consumed; and the 
results are, that every grain of the vapour taken up by a foot of common twelve-candle 
gas increases its light about 10 per cent. The various naphthas of commerce are more or 
less charged with benzole and its homologues, and they raise the illuminating power of 
gas from 4£ to 8 per cent, for each grain of the vapour absorbed by the gas. In the act 
of burning, a cubic foot of benzole vapour consumes 7| cubic feet of oxygen, or 37i 
cubic feet of air; and it produces 6 cubic feet of carbonic acid, and much aqueous 
vapour. The heat of the flame also is considerable. Benzole is but slightly absorbed 
by water, but is freely taken up by alcohol, ether, and the volatile and fixed oils. It is 
also, like the other rich hydrocarbons, absorbed by vulcanized tubing. The vapour is 
slowly condensed by chlorine and bromine in the sunlight, and compounds are formed 
containing six proportions of the halogens (C 12 H 6 Cl fi and C 12 H 6 Br 6 ), which present a 
certain analogy to Dutch liquid. Sulphuric acid also absorbs the vapour, and forms 
conjugate acids; but the most interesting product of benzole is its substitution compound 
with peroxide of nitrogen, which is produced when it is brought into contact with 
strong nitric acid. This, indeed, is the test for benzole ; so that if coal gas is passed 
through fuming nitric acid, as you here see, the benzole vapour is absorbed, and an oily 
liquid is produced, which has the odour of bitter almonds. This is easily purified by 
washing it with water, and finally with a weak alkaline solution. It is then called 
Nitro-benzole, or Essence of Mirbane. It is produced from benzole by the substitution 
of one proportion of peroxide of nitrogen for one of hydrogen (C 12 H 5 N0 4 ) ; and I may 
remind you that, although it is used very largely in perfumery, it is a dangerous poison; 
I know instances where a few drops of it have produced fatal coma. Benzole vapour is 
decomposed by a red heat, forming a gaseous hydrocarbon, and depositing much carbon. 
It is also easily condensed by cold, when it forms an ethereal liquid, which freezes into 
a crystalline solid at 32°, and which boils at 177° Fahr. This liquid is lighter than 
water, and is remarkable for its solvent power for caoutchouc, gutta-percha, and all 
kinds of resins and fats. 
(g) The other members of the benzole series, as Toluol (C 14 H 8 ), Xylol (C 16 H ]0 ), 
Cumol (C 18 H 12 ) and Cymol (C 20 H 14 ), are no doubt also present in coal gas, for they are 
found in the naphtha distilled from coal tar; but as they are less volatile than benzole, 
they are present in smaller quantity. 
(h) Naphthalin (C 20 H s ) is the last of the hydrocarbons to which I shall refer. It is 
only present in gas which has been made at high temperature, and it is invariably a 
secondary product of the decomposition of the richer hydrocarbons (tar, etc.) by the 
walls of the red-hot retort. In London, where it is the practice to work at very high 
temperatures, the coal gas is always charged with naphthalin, and its presence in the 
mains is a serious inconvenience. The vapour of naphthalin is very rich in carbon—a 
cubic foot of it contains five times its bulk of carbon vapour, and four times its bulk of 
hydrogen. The specific gravity of the vapour is 4-422, and therefore 100 cubic inches 
of it weigh rather more than 137 grains. It burns with a bright sooty flame, and if I 
heat a little of the hydrocarbon in a flask, and pass hydrogen gas through it, you will 
see how richly it naphthalizes the gas. In the act of burning, a cubic foot of the vapour 
consumes twelve times its volume of oxygen, sixty times its bulk of air, and it forms 
10 cubic feet of carbonic acid and aqueous vapour. Naphthalin is not absorbed by water, 
but it is by the volatile and fixed oils, and by naphtha and coal-tar; hence the advantage 
of keeping the raw gas until it is thoroughly cooled in contact with tar and ammoniacal 
liquor in a long hydraulic main. Hence, also, the fact that the richer qualities of gas, 
containing much hydrocarbon, do not deposit naphthalin; and hence, also, the use of 
naphtha as a remedy for naphthalin in the mains and service-pipes. The vapour of naph- 
