1006 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
increased in a similar manner, for the reason that during 
the precipitation into the interior of the flame of the 
solid hydrocarbons—viz., during the decomposition, under 
the influence of the high temperature of the flame of 
the gaseous bodies—small quantities of very much more 
highly condensed substances are formed, substances which 
probably would exist in the solid state if isolated at 
ordinary temperatures. Further than this, coal-gas, al¬ 
though practically a gaseous body at all temperatures, 
does, as has been said, act as a vehicle for holding in 
suspension, and carrying a certain quantity of the vapours 
of such bodies as benzole, toluole, naphthalin, etc., which 
are to be found in the tar, with what practical success— 
.success which is sufficiently certain as to be relied 
upon—we shall have to consider when speaking of the 
special carburation of gaseous bodies as a means of ob¬ 
taining illuminating gas. It is, indeed, for this purpose 
that these remarks regarding the luminosity of flame have 
been introduced. 
As there are at least three schemes, to which reference 
will be made, for the manufacture of gas, which deal with 
modifications of the existing apparatus employed, it will 
be desirable to give a general statement of that which is 
at present almost exclusively used. 
I he coal to be distilled is exposed, in quantities of about 
five hundred-weight at a time, to a comparatively high 
heat—a bright cherry-red heat is usually employed—in 
elongated tubular vessels of earthenware or iron, called 
“ retorts,” the shape of which varies slightly, some being 
that of a capital D, with its perpendicular stroke placed 
as a base, others circular, and others elliptical; ten feet 
long, twenty inches wide, and fourteen inches high, are 
about the proportional measurements. A number of such 
retorts are usually set in a furnace, and heated by one 
fire ; in most large works double retorts are used, which 
are worked from both ends, as if they were two single 
ones. At the mouth, or mouths, is a flange, upon which 
a plate of metal the lid—is capable of being secured 
strongly, and made tight with luting, if necessary • through 
these mouths the coal is introduced by means of long scoops 
or gutters, and the coke at the close of the operation with¬ 
drawn by the aid of rakes. Immediately upon the intro¬ 
duction of a charge of coal into a heated retort, carbonization 
commences, and gas is evolved. This carbonization occurs 
first with that portion of the coal which lies in contact with 
the sides and bed of the retort, and hence is more complete 
than that of the interior, which, however, undergoes de¬ 
structive distillation, and furnishes gases and vapours of 
higher hydrocarbons. These coming into contact with 
the. heated mass on the exterior, split up into simpler 
todies, the great desideratum being to heat the coal to an 
equal extent all through at the same time. The gas is 
conducted from the retorts by perpendicular pipes, about 
five inches in diameter, which rise near the mouth, called 
stand-pipes ; these pipes, which taper towards their upper 
extremities, are, above the furnace, bent twice at a right 
angle, and enter a larger pipe running horizontally over 
the centre of the retorts. This tube, which is called 
the “hydraulic main,” is at all times half full of tar 
and oil, and the ends of the bent tubes are allowed to 
dip into this tar to the depth of two or three inches, for 
the purpose of cutting off all communication between the 
retort and the gas which has left it. This hydraulic main 
is fixed horizontally, the tar being drawn off from its ex¬ 
tremities, and as the whole length of tube is kept at a fair 
heat—which heat should not be too strong—by reason of 
its contiguity to the furnace, the tar is usually in a liquid 
condition. It becomes essential that the gas should be 
submitted to a process of cooling, in order that those more 
condensible bodies which have been vaporized by the 
great heat may be deposited; owing to the great mobility 
of. gaseous matter, and its possessing a high latent heat, 
this process becomes one of some little difficulty, and 
necessitates the exposure of the gas to a very large cool¬ 
ing surface. This is attained either by simply passing the 
gas through iron tubes of sufficient length, or through 
[June 14, 187:'.. 
hollow iron columns, having an annular closed space 
through which the gas circulates. The extent of surface 
needed is sometimes very large, as much as ten square 
feet of cooling surface being often necessary for every 
cubic foot of gas passing per minute. If it were left for 
the evolution of the gas in the retorts to maintain a suffi¬ 
cient pressure to drive it through the various apparatus, 
considerable loss would result from leakage ; and it is 
found further that the gas produced under pressure is not 
so good in illuminating power, a considerably greater 
quantity of carbon being deposited under such circum¬ 
stances. Hence is is customary to employ what is called 
an exhauster, this being usually a rotary fan, acting the 
part of a rough air-pump. The use of this apparatus 
places the pressure entirely under control, and allows it to 
be reduced at the retorts to about half-an-inch of water. 
After the gas leaves the condenser, it has to be sub¬ 
mitted to the various processes of purification; but as these 
do not in any way bear upon the present subject, it will 
be sufficient to state that it is first washed or scrubbed by 
passage through a coke tower, over which water is allowed 
to trickle, to remove any residue of tar and also the 
ammonia which it contains. Thence it passes into a tank 
of milk or cream of lime, kept in continuous motion for 
the removal of carbonic acid and sulphuretted hydrogen, 
and lastly through a series of trays filled with dry lime 
and oxide of iron, for the removal of the residual sulphur 
existing in the form of sulphuretted hydrogen or bisul¬ 
phide of carbon. The gas is then stored and distributed. 
It was mentioned in an earlier portion of this paper, 
that if coal be distilled at a high temperature, a very 
large yield of gas can be obtained ; this increased yield, 
however, will be coupled with a considerable diminution 
of illuminating power. The reason for this was then ex¬ 
plained—in fact, the distillation of coal may be carried on 
at such a low temperature that the quantity of gas 
obtained is practically nothing, the whole of the products 
appearing in the solid or liquid state (a prolonged exposure 
of gaseous hydrocarbons to a low or even moderately high 
heat is attended with precisely similar results, viz., the 
formation of more complex substances), or conversely the 
heat may be so high as to completely decompose the 
whole of the hydrocarbons into their constituent elements. 
It is found advisable, in ordinary working, to take a mean 
of these results, and thus a fair yield of gas is obtained, 
together with a not inconsiderable amount of tar and oily 
matter. These latter products increase in direct propor¬ 
tion to a decrease of temperature, and a consequent de¬ 
crease of gas—thus : 
Table of Amount of Gas and Tar obtained at Different 
Temperatures. 
Temperature of Distillation. 
Amount of Gas 
obtained. 
Amount of Tar. 
Very low red heat ... 
7500 
150 lbs. 
1300° to 1400° F. ... 
8300 
120 lbs. 
Bright red heat . 
9500 
70 lbs. 
There are three considerations which influence a gas 
engineer with regard to the temperature at which he shall 
distil the coal, and which sometimes compel him to use a 
heat higher than would otherwise be advantageous ; these 
are, first, the time which can be afforded for the produc¬ 
tion of a given quantity of gas—a lower temperature 
means a more or less lengthened time of exposure for the 
same quantity of coal, and where, as is usual, the neces¬ 
sary plant is only just sufficient for the supply of the 
required quantity of gas, this prolongation of the process 
is impossible ; second, the percentage of fuel (coke) used 
in the furnaces for the distillation, this is likely to be 
greater for a greater interval of time ; and, thirdly, the 
quality of the coke produced, for the demand for coke, 
and consequently its value, is great, and it is therefore the 
