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 deg. to 500 deg.  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 deg.  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 better are the results.  In two retorts of the same size, one making 5,000 and the other 10,000 cubic feet per day, the gas will be twice as long in contact with the surface of the retort in the former as in the latter—­to the probable detriment of its quality, and increased tendency to stoppage in the ascension pipes.

A subject closely allied to that just alluded to is the temperature of the gas as it leaves the retort.  Until within the last few years, it was generally assumed that this was not higher than from 200 deg. to 300 deg.  Fahr.; and a very plausible theory was given to account for such a comparatively low temperature.  A discussion which took place a few years ago in the Journal of Gas Lighting showed that at that time opinions on this subject were not unanimous.  But the conclusion arrived at seemed to be that the gas was not higher in temperature than that before stated; and if higher temperatures were observed, they were due to the tarry matter in the gas, and were not those of the gas itself.  A little reflection is sufficient to show that the existence of gas intimately mixed with tarry matter at a high temperature, without being itself raised to that temperature, is a physical impossibility.

In a paper read to a Continental gas association about a year ago, the writer stated, as the result of many experiments, that unless the temperature in the ascension pipe rises above 480 deg.  Fahr., thickening of the tar in the hydraulic main and choking of the ascension pipe will certainly occur.  This led me to make a series of experiments, extending over many months, on the temperature of the gas in the ascension pipes