30 Drs Christison and Turner on the Construction of 
and, in consequence, most of the particulars of its manufacture have been set- 
tled with great precision. The richest material, the proper form and size of 
the retorts, the best method of applying the heat, the due degree of tempera- 
ture, the best construction of the apparatus for condensing the contaminating 
ingredients, have been separately made the subject of long, frequent, and most 
careful investigation ; and little difference of opinion now exists on any of 
them. 
Very different is the case with oil-gas. An idea generally prevails, in- 
deed, among manufacturers, and has even been adopted by some men of science, 
that the quality of oil-gas is not liable to the same variety as that of coal-gas. 
This notion appears to have originated in the fact, that the material has been 
always pretty nearly the same. The similarity of the material would natu- 
rally lead, cceteris paribus , to a uniformity in the quality of the gas in different 
places ; and this consideration is an important one, in regard to the relative 
economy of the two gases ; because, on account of an unavoidable difference 
of material, the quality of coal-gas, however well made, and however much 
the mode of manufacture may be hereafter improved, must always continue 
very different in different places. 
But there is another cause of variety besides difference of material, name- 
ly, variations in the mode of manufacture ; and, contrary to the general opi- 
nion, the mode of manufacture has appeared to us both more delicate, and less 
perfectly understood in the case of oil-gas, than in that of coal-gas. That it 
is less perfectly understood, will appear from the consideration, that no expe- 
riments sufficiently extensive and accurate have been hitherto published re- 
garding the circumstances in the manufacture of oil-gas, which alter its qua- 
lity, — that almost every point to be attended to in its manufacture, continues 
on that account unsettled, — and that great differences prevail among the pro- 
cesses followed in different places. While the method of making it continues 
thus unsettled, the quality of the gas must be very precarious, and, conse- 
quently, the whole process will appear to be one of great nicety. Of this no 
better proof can be furnished, than a fact which has come under our notice, 
namely, that the gas made by the same workman, under circumstances to all 
appearance the same, and generally of very high specific gravity, was once so 
low as 773. 
Besides these causes of variety in the quality of the gas, we believe we may 
likewise add differences of material. This circumstance, indeed, has little in- 
fluence at present, and has perhaps even less than is generally imagined. Gas 
of very different qualities has been made from whale-oil, cod-oil, palm-oil, 
cocoa-nut-oil,, and linseed-oil. But as these oils do not differ very materially 
from one another m chemical composition, we suspect that the gas would be 
nearly the same from all, if the process was properly adapted to each. And, 
in point of fact, the differences observed among such gases are not nearly so 
great as those which exist among different samples of gas made apparently 
with great care from good whale-oil. It is very likely, however, that methods 
may be discovered for improving the quality of oil-gas, by the addition of 
other substances to the oil. 
Whatever weight be attached to these explanations, there can be no doubt 
of the fact, that the quality of oil-gas does vary most materially, — out of all 
