June 14, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
1005 
will immediately be depressed to a certain extent, which 
depression will be due to the vapour given off from the 
water at the particular temperature at which the observa¬ 
tion is made, and its amount will represent the vapour 
tension of water. Now, if alcohol be used in a similar 
experiment, the depression will be much greater, and 
hence the vapour tension of alcohol is said to be greater 
than that of water, and so on, every liquid giving a 
different result. 
Now, if we take a case in which the amount of liquid 
passed to the top of the mercurial column is insufficient 
for the formation of that amount of vapour which the 
given space can take up, the whole of the liquid will then 
disappear, and the depression will appear to be due to the 
presence of a quantity of true gas, and this is borne out by 
the fact that a vapour in this state—unsaturated, as it is 
called—follows all the laws that the more permanent gases 
do. It will be found, for example, to vary in volume 
directly as the pressure, and further to increase or decrease 
in volume at a uniform rate on an increase or decrease of 
temperature. But now, on the other hand, let more of 
the liquid be passed up into the vacuous space than can 
possibly be required to fill it with vapour ; in this case a 
quantity of the liquid will remain upon the surface of the 
mercury, and the superincumbent vapour is then said to 
be saturated, which indicates that as much vapour as can 
exist at that particular temperature will be found so ex¬ 
isting in the given space ; if, therefore, the temperature 
be now raised, a further quantity of the liquid will be 
converted into the gaseous condition; if lowered, a por¬ 
tion of the liquid already existing as gas will return to the 
liquid state ; in the same way, if the pressure be lowered 
or raised, the same result will be observable. Thus it is 
seen that the vapour proceeding from every liquid sub¬ 
stance possesses a fixed maximum tension for every par¬ 
ticular temperature or pressure, which it cannot exceed 
under any circumstances. 
It follows, also, that if we take the case of an unsatu¬ 
rated vapour, by either increasing the pressure or reducing 
the temperature, we shall eventually reach this point of 
maximum tension or saturation, after which any further 
increase of pressure, or decrease of temperature, will be 
attended with a condensation of the vapour into the liquid 
state; and, in fact, it has been by using either one of these 
processes, or both combined, that nearly the whole of the 
bodies existing as gases have been liquefied. Another 
point must be mentioned here. In the above illustrations 
the various liquids were supposed to be passed up into a 
vacuum, the vacuum existing above the mercury, in an 
ordinary barometer. Now, precisely the same results 
would have been obtained if this vacuous space had been 
filled by any gaseous body ; the quantity of vapour given 
off by a liquid is absolutely independent of the medium 
into which it is so given off. 
The importance of some of these facts, as regards the 
manufacture of gas for illuminating purposes, will be seen 
when it is considered that coal gas not only contains cer¬ 
tain permanent gases, but also a large quantity of the 
vapours of volatile hydrocarbons, which contribute in no 
small degree to its luminosity ; and these points will be 
referred to again, more particularly when speaking of 
carburetting air or gas. 
A few words are necessary here to state a few facts 
regarding the luminosity of the flame of gaseous hydro¬ 
carbons. 
It is still generally accepted that the luminosity of flame 
is due to the presence of intensely heated solid particles 
(although some powerful objections have been urged 
against such a belief). In the case of the combustion of 
all hydrocarbons, these solid particles are particles of 
carbon. 
Now, a flame will be more or less luminous according 
to the greater or less number of such particles present, 
and to the temperature to which they are raised. The 
denser the hydrocarbon, the greater number oi carbon 
particles will exist in a given space, and when its vapour 
is burned there will be a correspondingly greater number 
precipitated into the interior of a flame. Now, bearing 
these two points in mind, it will be manifestly advantage¬ 
ous to obtain for combustion such compounds of carbon 
and hydrogen in that proportion of the two which shall 
give us the greatest quantity of carbon to the smallest 
quantity of hydrogen, provided that the heat of the flame 
is maintained sufficiently high to raise the carbon particles 
to whiteness. This latter condition is quite as important 
as the former, for it is quite possible that a gas containing 
a considerable amount of the denser hydrocarbons shall, by 
reason of the comparatively low temperature of its flame, 
possess less illuminating power than a gas possessing a 
smaller quantity of such hydrocarbons, the temperature of 
the flame of which, however, is somewhat higher. It 
would appear to follow from this, that every effort ought 
to be made by the gasmaker to obtain a gas containing as 
large a proportion as possible of these more luminous 
compounds ; but a glance at the following table, contain¬ 
ing analyses of the gas supplied by several different com¬ 
panies, will show that, at any rate, this effort, if made, is 
not very successful, the greater bulk of the gas being 
composed of hydrogen, the flame of which is without any 
light-giving properties whatever, and marsh gas, a gas 
possessing the least light-giving power of any hydrocarbon, 
as it contains the smallest proportion of carbon to the 
largest of hydrogen. 
Composition of Coal Gas Supplied by the Following 
Companies :— 
Great 
Central. 
Imperial. 
Chartered. 
i Illuminating hydrocarbons 
Marsh gas. 
Hydrogen. 
Carbonic oxide .... 
Carbonic acid. 
Nitrogen.. 
Oxygen. 
3-56 
35-28 
51-24 
7-40 
0-28 
1-80 
0-44 
3-67 
40"66 
4T15 
8-02 
0-29 
5-01 
1-20 
3-53 
35-26 
51-80 
8-95 
0-38 
0-08 
100-00 
100-00 
100-00 
The result, as stated here, is even more unfavourable 
than it at first appears, for the above figures, representing 
the quantity of illuminating hydrocarbons, express not 
only those that are really gaseous bodies, but also the 
vapours of liquids held dissolved in the gas. There can be 
no question that a great advance in the manufacture of 
coal-gas would be made, if by any means, applied either to 
the primary distillation of the coal or to the gas after its 
production, whereby the amount of acetylene (C 2 H 2 ) or 
of olefiant gas (C 2 H 4 ) could be increased ; for olefiant 
gas contains, in a given bulk, just twice as much carbon 
as marsh gas, and acetylene a still larger quantity. The 
value of this increase is not to be measured only by the 
increased quantity of carbon contained in a given bulk of 
gas, for a small portion of either of the above compounds, 
diffused through a non-illuminating gas, is much moi e than 
equivalent to a similar quantity of carbon combined in 
some less condensed compound. An illustration of this 
occurs in the case of marsh gas. If marsh gas, possessing, 
it will be remembered, only a small amount of luminosity, 
be decomposed, either by its passage through a heated 
tube, or by the electric spark, the carbon it contains will 
be deposited, while the amount of hydrogen set free will 
occupy double the volume of the original gas. Yet the 
flame of this nearly pure hydrogen will be found to possess 
a greater luminosity than the flame of the original marsh 
gas, although it has lost nearly the whole of its light-giving 
material, accounted for by the presence of a very small 
quantity of acetylene, produced during the decomposition. 
The light of a coal-gas flame is, without doubt, somewhat 
