An important subject for investigation, which has not yet been
satisfactorily determined, is the temperature at which it is most
beneficial to distill coals of various qualities. The practice of
allowing the charge to remain in the retort for some time after most
of the gas has been driven off, to enable (it is said) the retort to
recover heat for the next charge, often leads to misconception as to
the true temperature of carbonization. The effect of this is to
equalize the temperatures inside and outside the retort. This inside
temperature is not maintained, the temperature outside not being high
enough to transmit the heat with sufficient rapidity; and so, in an
apparently hot retort, the coal may be carbonized at a comparatively
low temperature. A truer test of temperature is that of the outside of
the retort, which should be not less than 400° to 500° Fahr. above the
temperature necessary for proper carbonization. In all experiments
relating to temperature pretending to any degree of accuracy, a
pyrometer of some kind should be used. Judging of the temperature by
the color is often misleading. Not only may the eye be deceived, but
different clays do not present the same appearance at the same
temperature. A good, reliable pyrometer to estimate temperatures to
(say) 2500° Fahr. is much wanted.

Experience during the last few years with the high temperatures
obtained by the use of regenerative furnaces has led me to the
conclusion that higher heats than are usual may be employed with
advantage, as regards both the quantity and the quality of gas,
provided the retorts are heated uniformly throughout their length, and
the weight and duration of the charge are so adjusted that the coal
does not remain longer in the retort than is just sufficient to drive
off the gas; and that the more rapidly the coal is carbonized, the