SPATE GHOLOGISM. 303 
proper jacketing the upper part of the kiln, the losses due to radiation 
could be reduced sufficiently so that the extra length of kiln would not 
give rise to excessive losses by radiation. In Germany rotary kilns are now 
in use, according to Michaelis, 90 feet long, which turn out, by the dry pro- 
cess, 300 barrels per day with a fuel consumption of from 60 to 75 pounds 
ef coal per barrel, which is from 15 to 18 per cent. of the weight of the 
cement. The Edison kiln, 150 feet long, is 9 feet in diameter and is con- 
structed of cast iron in place of steel and its fuel consumption probably 
approaches quite closely 50 pounds of coal per barrel. At the time the 
writer visited the Edison works, before they were in operation, a capacity 
of 700 barrels per day was expected from each kiln, based on extensive 
preliminary experiments. 
From the theoretical standpoint, the lengthening of the kiln seems 
at. present to be the most efficient and simplest method of reducing the 
losses of heat and probably will also be the direction in which the kiln 
is improved. All other expedients for utilizing the waste heat are 
secondary and are bound to result in a far smaller economy. If in a 
longer kiln, the gases are cooled to too low a temperature, mechanical draft 
must replace the stacks. For the wet process, however, the length of the 
kiln is limited by the dew point of the water-laden gases which are pass- 
ing out from the kiln. Thus a go foot kiln having a capacity of 200 barrels 
per day, using a slurry containing 50 per cent. of water and consuming 
120 pounds of coal per barrel, should not discharge the waste gases at a 
temperature below 85° C. If the temperature is lower, the kiln is too 
long. It has been assumed in this illustration that only the amount of 
air theoretically required for the combustion of the coal is introduced. 
Sometimes the statement is made that in burning by the wet process it 
is necessary to use excess of air, as the theoretical volume of air could 
not carry the great volume of steam evolved from the slurry. This evi- 
dently is not true; in fact, with the wet process, it is necessary to use as 
little air as possible in order to maintain the stack temperature high 
enough for sufficient draft. Taking it altogether, there is no serious 
complaint to make against the heat wasted in the rotary kiln fed with 
slurry. As long as the manufacturer starts with materials containing 
naturally much water and is compelled to evaporate this water, this 
might as well be done in the kiln as anywhere else, as long as it does 
not cause serious trouble there due to organic matter, forming of 
“rings,” etc. Marl containing not more than 15 to 25 per cent of water 
should not be worked wet, but should be dried in the rotary dryer in the 
beginning: of the process and then worked like any other dry mixture. 
Marl excessively high in organic matter likewise should be dried previous 
to grinding and burning. ‘This is due to the fact that in the wet rotary, 
