27 
It is essentially the decomposition of substances, solid or 
liquid, by heat, in vessels from which the air is excluded, so as 
to obtain gaseous, liquid, or solid products. 
Generally the substances used are of organic origin, and 
the bodies evolved are dependent —(1) upon the nature of the 
material; (2) upon the degree of heat applied; and (3) upon the 
nature of the atmosphere within the still and condenser, and 
upon the extent of the cooling. 
In regard to the first of the foregoing principles—viz., the 
nature of the material subjected to heat—it is necessary to point 
out that the working is affected primarily by the chemical com- 
position; and, secondarily, by the mechanical condition. The 
chemical composition of most of the organic substances used 
in these industries is a modification of woody fibre in which 
(C,H,,0;),—cellulose—is the essential component. 
Percentage Compositions of 
—— 
la a = 
Organic Portion - . 
| Geuniose. of Bituminons {ylgamcanne) Anthracite 
Carbon .... 44444 ., 73:05 .. 85°81 .. 94:05 
Hydrogen .. 6173 .. 1062 .., a85 Ct 3°38 
Oxygen .... 49383 .. 1633  .. 834... 2°57 
100°000 ~=.- 100°00 .. 100:00 .. 100:00 
The results obtained in actual working are, however, modified 
(as before mentioned) by the mechanical condition. Sometimes 
the organic matter is associated with argillaceous or aluminic 
constituents, which render it necessary to very much increase the 
temperature applied to the retorts. The shale used by the oil 
manufacturers of Scotland is a good example; the infusible or 
non-volatile matter absorbing a very large proportion of the heat 
which is applied. This leads us to consider our second general 
principle—viz., the effect of temperature. 
All chemical action has an equivalent or measure in quantity 
and degree of heat. (Quantity of heat is represented by “ units 
of heat ;’’ degree, or intensity of heat, is represented by ‘ tem- 
perature.”’) Chemical combinations of matter give out heat in 
definite proportions; and the inverse action, or decomposition, 
requires at least an equal expenditure of heat-energy. So much 
is this the ease, that it is necessary for the chemist, in every 
instance, to know the condition of temperature before he can say 
whether a combination or decomposition will take place in bodies 
under consideration. 
At a very high temperature the products from coal or shale 
are carbon, and much carbonized gas of low illuminating power, 
with but little liquid distillate. At a low temperature there is 
much liquid product and little gas, but of high illuminating 
