In general, the object is to study the process of combustion of coal. When soft coal is burned in any furnace, part of the combustible is driven off shortly after charging, and has to be burned in the space between the fuel bed and the exit of the gases, which is called the combustion space. There is enough evidence to show that, with a constant air supply, the completeness of the combustion of the volatile combustible depends on the length of time the latter stays within the combustion space; but, with a constant rate of charging the coal, this length of time depends directly on the extent of the combustion space. Thus, if the volume of the volatile combustible evolved per second and the admixed air is 40 cu. ft., and the extent of the combustion space is 80 cu. ft., the average time the gas will stay within the latter is 2 sec.; if the combustion space is 20 cu. ft., the average time the mixture can stay in this space is only ½ sec., and its combustion will be less complete than in the first case. Thus it is seen that the extent of the combustion space of a furnace is an important factor in the economic combustion of volatile coals. The specific object of the investigations, thus far planned, is to determine the extent of the combustion space required to attain practically complete combustion when a given quantity of a given coal is burned under definite conditions. With this object in view, the furnace has been provided with a combustion space large enough for the highest volatile coals and for the highest customary rate of combustion. To illustrate the application of the data which will be obtained by these experiments, the following queries are given: Suppose it is required to design a furnace which will burn coal from a certain Illinois mine at the rate of 1,000 lb. per hour, with a resulting temperature of not less than 2,800° Fahr. How large a combustion space is required to burn, with practical completeness, the volatile combustible? What completeness of combustion can be attained, if the combustion space is only three-fourths of the required extent? In the present state of the knowledge of the process of combustion of coal, these queries cannot be answered definitely. In the literature on combustion one may find statements that the gases must be completely burned before leaving the furnace or before they strike the cooling surfaces of the boiler; but there is no definite information available as to how long the gases must be kept in the furnace or how large the combustion space must be in order to obtain practically complete combustion. It is strange that so little is known of such an old art as the combustion of coal.