■June 21, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
1021 
of higher illuminating power, the amount of tar being 
correspondingly less ; the difficulties encountered appear, 
as in previous methods, to be chiefly mechanical, the great 
friction to be overcome by the working of the screw re¬ 
quiring considerable power, the wear and tear thus be¬ 
coming also greater. Further, the exposure of the iron to 
such a continuous high temperature renders it extremely 
brittle, a quality enhanced no doubt by the presence of 
sulphur, which causes a superficial conversion of the iron 
into sulphide. That the process itself is to a great degree 
successful, wili be seen from the following results :— 
Coal 
Carbonized. 
Quantity of 
Gas obtained. 
Illuminating 
Power of Gas. 
Amount of Tar. 
1 ton 
11,040 
16 candles 
S'5 gallons. 
1 ton 
10,160 
17 candles 
— 
1 ton 
12,000 
16 candles 
—*— 
The fact of the existence of a large number of hydro¬ 
carbons of great volatility, the vapours of which, when 
ignited, burn with exceedingly luminous flames, has been 
a very great incentive with inventors to produce some 
scheme whereby these vapours might be used to confer 
luminosity upon a combustible non-luminous gas, or fur¬ 
ther, inflammability, coupled with luminosity, upon a non¬ 
combustible and negative gas. The Patent Office contains 
the record of many such schemes, which, for some reason 
or other, have failed to fulfil their originators’ hopes and 
desires. Much misapprehension has arisen upon this 
matter. Too much has often been expected from such 
schemes; but, on the other hand, a large amount of un¬ 
reasonable contempt has been poured out upon all projects 
of the kind, both extremes being avoidable by a short con¬ 
sideration of certain well-known physical laws. It should 
be remembered by the opposers of the possibility of there 
being any good in such ideas, that to a great extent, larger 
than is generally thought, the luminosity of coal gas itself 
is dependent upon the presence of .the vapours of such 
volatile substances, which are held in a state of solution 
in the gas, but which still remain condensible bodies; and 
we shall see presently that the loss in illuminating power 
which coal gas undergoes on exposure to a low tempera¬ 
ture, is really due to the condensation of such substances. 
These remarks apply to the principle of carburation, and 
not to any special means of applying that principle. 
This idea of carburation is not by any means new, 
having been started almost at the same period that light¬ 
ing by coal gas in the ordinary way was introduced ; but 
it was not until the opening up and working of the Ame¬ 
rican oil wells, by which a supply of very light petroleum 
oil or spirit was obtainable, that any scheme in this direc¬ 
tion was found to hare any chance of success. In 1862 a 
patent was taken out in France by a M. Mongruel, which 
proposed to pass air through an exceedingly light and in¬ 
flammable spirit, the air thus becoming charged with a 
certain amount of a highly luminous hydrocarbon vapour, 
in such a quantity as to render the mixture inflammable, 
and to allow of its being burned as an ordinary combus¬ 
tible gas. This idea was introduced into England by a 
company known as the Photogenic G-as Company, which 
received some amount of support at the time, but, from 
.some cause or other, did not prove a success. 
The various inventions brought into notice since that 
time for the purpose of carburetting air and other gases 
have been based upon precisely the same principle, but 
have differed considerably in the apparatus employed. 
A general distinction may be made here—more for the 
sake of separate examination than from there being any 
fundamental difference between them — between those 
processes which propose to use air as the vehicle or me 
dium for holding in solution the hydrocarbon vapour, and 
those in which a combustible but non-luminous gas acts 
in such a manner; in either case any remai’ks upon the 
quantity of vapour taken up, and its character, when 
held in solution, will apply to both. Many erroneous 
statements have been made on this matter for the want 
of a little scientific knowledge; for instance, in some cases 
patentees themselves have declared that a union takes 
place, other than mechanical, between the gases of the air 
and the vapours of certain hydrocarbons, which results in 
the production of such gases as olefiant gas, or acetylene, 
an utter absurdity and impossibility which would be evi¬ 
dent to any who would consider under what circumstances, 
and under what circumstances only, such gases are pro¬ 
duced. A vapour of any hydrocarbon whatever will, 
whether it exist by itself or whether it be diffused through 
another gas or gases, always be such a vapour, and will 
invariably exhibit the properties characteristic of it; and, 
moreover, if diffused through a gas, will be found con¬ 
densible once more to the liquid from which it was formed, 
if proper means be applied. An illustration occurs in the 
case of aqueous vapour, which always exists in greater or 
less quantity diffused through the air ; but this vapour is 
still the vapour of water, exhibiting all its properties, and 
capable once more of assuming the liquid state on a suffi¬ 
cient reduction of temperature. Another view equally 
erroneous is, that a given bulk of air or gas will take up 
and hold in solution an almost indefinite quantity of va¬ 
pour, and some extraordinary statements have been made 
as to the large amount of this quantity. Now, from what 
has been said in the earlier portion of this paper, it will 
be readily understood that this point is capable of experi¬ 
mental proof, and is not determined either by chance or 
accident. Every liquid body has a maximum tension of 
its vapour, and this point is as fixed for any particular 
substance as its specific gravity or its boiling point. If, 
therefore, a given space, filled either with air or other gas, 
contain a quantity of vapour at its maximum tension, that 
space will be saturated with such vapour, and it will be 
found impossible to cause it to hold ever so slight a quan¬ 
tity in excess of that already existing; as, however, this 
tension is lower for lower temperatures, a space that is 
saturated at a higher temperature will become over, or 
supersaturated as the temperature falls, and hence must 
deposit a quantity of vapour in the liquid state. When 
experiments are made with a view to test this result, the 
temperature of the gas should be the observed tempera¬ 
ture and not the temperature of the vessel containing it, 
as the conducting power of a gas for heat is so bad that it 
takes a considerable time for any quantity of it to fall 
from a higher to a lower temperature. It will be seen, 
then, that the fixing of the maximum tension of the 
vapour from a liquid for different temperatures, will give 
us an exceedingly good indication of the quantity of the 
vapour which may exist diffused through any space of air 
or other gas. A statement was made at the outset, which, 
for the sake of clearness, is repeated here, viz., that the 
medium in which the vapour is diffused has no effect 
whatever upon the quantity so diffused, a vacuum, or 
equal volumes of all gases, taking up the same quantity 
of the vapour of any liquid. The boiling-point oi a liquid 
is far more intimately connected with its vapour tension 
than its specific gravity ; indeed the boiling-point depends 
upon the vapour tension, for when this tension equals in 
amount the pressure of the atmosphere, a body is said to 
boil, and does at that point enter into ebullition. A great 
tendency has been shown by inventors to reduce the spe¬ 
cific gravity of the hydrocarbons which they use, with a 
view to obtain bodies of very great volatility, and to some 
extent the boiling-points of such bodies have been disre¬ 
garded; probably this has not led them much astray, 
because, as it happens, the substances mostly used for 
these purposes, the light petroleum spirits, have low spe¬ 
cific o-ravities and low boiling-points, the two being i educed 
at the same time, but if almost any other class of hydro¬ 
carbons had been dealt with, the specific gravity would 
have proved no guide to the volatility of the compound , 
thus, in the case of the following bodies, alcohol, benzole, 
and’bisulphide of carbon, which have respectively the 
