70 On Oil and Oil-gas. 
The gas from oil appears to be variable in 
its qualities; partly, I apprehend, from the 
different species and qualities of the oil, but 
chiefly from the variations in the mode of ob- 
taining it, and the degrees of heat at which 
the decomposition is effected.. We learn from 
Dr. Henry’s experiments on coal gas (Philos. 
Trans. 1808,) that the best coal gas comes 
over inthe early part of the distillation, and be- 
fore the greatest heat is applied to the retorts. 
The same seems to obtain in the decomposition 
of oil; and it seems probable that the gas 
will be the best which is obtained with the 
most moderate heat, all other circumstances 
being the same. From the experiments al- 
ready detailed it is evident that the heat for 
decomposing oil must considerably ‘exceed 
700°, as it sometimes rises to that point or 
near it in the process of distillation, in which 
very little, if any gas is produced. <A heat 
visibly red at least seems required for the de- 
composition of oil into a permanent elastic 
fluid. ' 
The usual mode of decomposing oil is I 
believe, to let the oil descend by drops into 
an iron retort heated red, in which is aquan- 
tity of some substance having little or no 
chemical action upon oil, such as charcoal, 
pounded bricks, &c. The object ‘of which 
