the action of which depends on the pressure produced by the
vaporization of mercury in a malleable iron tube. The end of this tube
was first rested on the top of the coal, but not in contact with the
retort. It reached about 18 inches into the retort, and therefore was
not in the hottest part. In this position the temperature indicated
shortly after charging the retort was 1110° Fahr., gradually rising to
1640° Fahr. The end of the tube was then embedded in the coal, when
the pyrometer indicated a temperature of 1260° Fahr. within 30 minutes
after the retort was charged; gradually rising toward the end of the
charge as before. At the time these temperatures were taken, the
retorts were each producing 10,000 cubic feet of gas per day. I had no
opportunity of testing the accuracy of the statement that, with lower
temperatures, there is a tendency to stoppage of the ascension pipes;
but with these high temperatures (contrary to what might be expected)
there is no trouble from stoppages.

These experiments, so far as they have gone, lead to the conclusion
that the temperature of the gas as it is evolved from the coal is not
less than 1200° Fahr., and that cooling commences immediately on the
gas leaving the retort. The temperatures being far above that of
liquefaction, the gases are cooled very rapidly. The temperature of
the gas in the ascension pipe depends on the rapidity with which the
gas is evolved--that is to say, the greater the quantity produced in a
given time, the less effective is the cooling action of the mouthpiece
and the ascension pipe; and although I had no opportunity of testing
it, I should expect to find that with retorts making from 5,000 to
6,000 cubic feet of gas per day, the maximum temperature in the
ascension pipe 18 inches from the mouthpiece will not exceed 400° to
500° Fahr., while with lower heats and lighter charges the
temperatures will be still lower. That these temperatures have some