1004 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[June 14, 1873. 
it is supposed that the wood or coal is heated in air, but 
it is evident that we may prevent the access of air to the 
heated material, and by so doing we can arrest the decom¬ 
position of such material before the final stage of decom¬ 
position, as the only oxygen present will be that given off 
from the substance, which will in either case be insufficient 
for the complete combustion of the carbon and hydrogen. 
If the substances be heated under such circumstances, a 
number of compounds of carbon and hydrogen will be 
formed, simpler than the original wood or coal, but still 
having a considerable complexity of constitution. 
This resolution of complex bodies to simple forms, under 
the influence of heat, out of contact with air, is termed 
destructive distillation, and is the process invariably em¬ 
ployed in the manufacture of gas directly from coal. 
Thus it will be seen that the possibility of making such 
gas depends in the first place upon two things :—- 
1st. The great complexity of organic bodies, especially 
vegetable matter, and consequently coal. 
2nd. The power which the elements constituting such 
substances possess, of forming a number of simpler bodies 
(some of them gaseous) when heated out of contact with 
-air. 
Although it is perfectly true that we cannot, in the 
process of destructive distillation, burn up our carbon and 
hydrogen into carbonic acid and water, for the want of 
the necessary oxygen, yet we may even go further than 
this, and reduce the carbon and hydrogen to their elemen¬ 
tary condition, if the heat be only sufficiently intense, and 
it may be mentioned here that the reason why the experi¬ 
ence of gas-engineers has always shown that a higher 
temperature gives a greater yield of gas per ton of coal, 
but gas of lower illuminating value, is to be found in this 
tendency of the carbon and hydrogen gradually to resolve 
themselves, under the influence of heat, into simpler and 
yet simpler products, until eventually a temperature is 
reached at which the greater portion of the carbon is de¬ 
posited, and the hydrogen goes off in its elementary 
gaseous state. 
The following Table, in which the proportion of hydro¬ 
gen is maintained at 100, illustrates this point. 
Table, showing the Proportion of Hydrogen and Carbon in 
Coal Gas distilled at different Temperatures. 
Temperature. 
Hydrogen. 
Carbon. 
Name of G-as. 
Dull red heat.. 
100 
614 
Principally olefiant gas (C 2 H 4 ). 
Red heat . 
100 
580 
Bright red) 
heat.j 
100 
472 
/Olefiant gas (C 9 H 4 ) mixed with 
( marsh gas 'CH 4 ). 
White heat. 
Continued / 
white heat, j 
100 
100 
325 
7 
Marsh gas (C H 4 ). 
(Nearly pure hydrogen, carbon 
\ deposited. 
The number of well-known intermediate products ob¬ 
tained during the destructive distillation of coal is very 
large, and it is extremely probable that there are many 
that have as yet escaped observation. The subjoined 
table enumerates the principal of these products, as well 
as the physical condition in which they exist at ordinary 
temperatures. It seems scarcely necessary to say here, 
that these are true products, and in no wise educts, that 
is to say, they had no previous existence in the coal. 
Hydrogen 
Marsh gas 
Acetylene 
Olefiant gas 
Propylene 
Gaseous. 
Butylene 
Carbonic oxide 
Carbonic acid 
Nitrogen 
Ammonia 
Water 
Benzole 
Toluole 
Cumol 
Liquid. 
Cymol 
Aniline 
. Picoline 
Bisulphide of carbon 
Solid. 
Paraffin 
Naphthalin 
Paranaphthalin 
Pyrene 
Chrysene 
These products are here represented as isolated and 
existing by themselves, but, in reality, they are found 
mechanically mixed in the rough materials obtained during 
gas distillation ; practically, the result of this distillation 
consists of only four products :—1st. The coke which re¬ 
mains upon the bed of the retort in which the coal is car¬ 
bonized ; 2nd. A light, watery fluid, which contains some 
of the more soluble gaseous substances dissolved in water; 
3rd. A pitchy or tarry substance formed of the liquid 
and solid products, the lighter portion of which contains 
the liquid oils and naphtha; and finally, the gaseous 
bodies, together with which is always found more or less 
of the vapours of the more volatile liquids. 
As the hydrocarbons become richer in carbon, and pro¬ 
portionately poorer in hydrogen, the tendency is for the 
substance to assume the liquid state ; and if this excess 
continues to increase, eventually to become solid. Thus, 
olefiant gas is C 2 H 4 ; benzole (a liquid), C 6 H 6 ; while 
naphthalin (solid) is C 10 H 8 . 
It is advisable here that we should look a little more 
closely into the physical condition of some of these bodies, 
as upon this depends the success, or want of success, which 
attends many of the new schemes. 
The solid, liquid and gaseous states of matter are not 
divided from each other by any sharp line of division, but 
gradually pass from one to the other by insensible grada¬ 
tions ; this is evident to us in the case of solids and liquids, 
for we are acquainted with many bodies which cannot 
fairly be placed in either class, and to which the term 
viscous may appropriately be applied ; not so evident, but 
still observable, is a state of matter bearing a relation to 
the liquid and gaseous states, such as viscous bodies do to 
liquids and solids ; and, from such and other evidence, we 
regard gases as being the vapours of liquid bodies, more 
or less removed from the boiling points of such bodies. 
A perfect gas would be defined as possessing the con¬ 
dition of perfect fluid elasticity, and presenting under a 
constant pressure a uniform rate of expansion for equal 
increments of heat, but it seems probable that this theo¬ 
retical definition is never absolutely realized, for although 
we still speak of a few gases as perfect, and represent 
them therefore as fulfilling the above law, yet all analogy 
and previous experience would indicate that eventually 
even this statement will have to be modified. The term 
vapour was for a long period a term for a distinct class of 
gaseous substances, viz. those which could be made to 
assume the liquid condition ; but, by the investigation of 
many experimenters, led by Faraday, this point is at pre¬ 
sent only a question of the adequateness of the means 
employed, and hence this term may now be used in a much 
wider sense, and may include all gases whatsoever, for 
that these are but the vapours of liquids possessing ex¬ 
ceedingly low boiling points appears to be distinctly 
proved. All liquids whatever, at all temperatures, give 
off certain quantities of vapour from their surfaces ; the 
amount thus given off differing for different bodies and for 
different temperatures ; if the liquid be enclosed in any 
vessel, the vapour will exert a certain pressure -upon the 
sides of such vessel, and this pressure will vary with the 
temperature, being higher for higher temperatures, and 
lower for lower ones. This pressure is termed the tension 
of the vapour of that particular substance. This may be 
readily illustrated in the following manner :—If a small 
quantity of water be passed up into the vacuum existing 
at the top of an ordinary barometer, the mercurial column 
