The Air-Bath and its Technical Equivalents

The same principles will hold good where larger quantities of material are used in the laboratory. For instance, in analytical work in which crucibles are employed, they are heated either with the lid cover on, to prevent access of air, or in a slanting position without the cover, so as to provide a draught of air through the mass. 

If we examine the apparatus which is used in works to produce results similar to those obtainable in the laboratory, we find that it is partly constructed on the same principles as those applied in blowpipe work, partly on the same principles as those applied in working with a closed or open crucible. A furnace which is used for operations in imitation of blowpipe work, that is, for work in which the mass to be heated is exposed to the direct action of the fire gases, is represented in Figs. 22 and 23. 

It consists of an arched chamber built of bricks and heated from a grate. The substance to be heated is spread on the bed of the furnace chamber the fire gases pass over it, and are taken out at the end opposite to the fire grate into the chimney. It is a matter of course that a furnace of this kind, when used for operations which require high temperatures, should be lined inside with fire bricks that is to say, that whilst it is sufficient to build the outside shell of the furnace, which adds to its stability and prevents radiation, with ordinary bricks set in mortar, it is necessary to use fire bricks set in clay, or other heat-resisting material, in those parts which are exposed to the direct action of the flame. It is evident that, according to the nature of the operation which is to take place, it will be necessary to alter the details of construe- 

Whilst we are dealing with this subject, we may point out that in the case of open roasters it may become necessary to consider the ingredients which are contained in the fire gases thus, coal which is rich in pyrites would not be suitable in operations in which the sulphurous acid produced would act on the substances operated on. Where such is the case, coke as free from sulphur as possible should be used, or better still a gaseous fuel. In the latter case the furnace will have to be constructed in such a manner that, instead of having a fireplace which is fed with solid fuel, it has a chamber arranged for the combustion of gas. 

All these furnaces are wasteful in the consumption of fuel, and wherever it is practicable the waste gases from them are used for other operations, as, e.g., for boiling down or drying. 

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