Destructive distillation consists in heating the fuel to temperatures of several hundred degrees, then a few of the constituents pass over undecomposed, while the bulk of the material undergoes thermal decomposition, the gaseous and liquid products of which are to be found in the form of gas and tar. It will be understood that by the application of a considerable reduction of pressure a larger portion of the less volatile constituents can be recovered unchanged in the distillate. It is likewise evident that too rapid a distillation at ordinary pressure will, by superheating of the walls of the carbonising vessel, lead to a secondary and unnecessary decomposition of the products which have already passed into the gaseous state.

The vacuum distillation of coal has been studied by Pictet in Switzerland, and by Wheeler in England, almost at the same time. It was found that by the use of a vacuum some of the petroleum-like hydrocarbons present in coal, which Pictet had isolated by extraction, could be obtained as distillates. For commercial purposes vacuum distillation will hardly come into question, firstly on account of the cumbersome apparatus required, and secondly because the same product can apparently be obtained by distillation at ordinary pressure with the aid of superheated steam.

My collaborators, W. Schneider and H. Tropsch, have investigated the vacuum distillation of lignite. They have found that, as against the liquid vacuum-tar of coal, a solid tar of as high a solidifying point as 53 degrees celsius is obtained, which consists of partly decomposed montan wax and a good deal of viscous oil. As regards the vacuum distillation of peat, little is known, but results similar to those from lignite are to be expected.

The distillation of fuels at ordinary pressure has been practised commereially for decades, on a very large scale, in gas-works, coke-ovens, shale and brown coal carbonising plants, not for the production, however, of a primary low-temperature tar (" Urteer "), but for other purposes. In all cases the desire for a rapid throughput, in view of the poor thermal conductivity of the fuel itself, is likely to lead to excessive heating of the walls of the carbonising vessel: There are also other reasons for this. In coking, the main object is to produce a firm and well-baked coke such as is required in the metallurgical industry.

In gas-works the use of high temperature is dictated by the aim at the highest possible yield of gas. In the brown coal distilleries, again, which have hitherto mainly been worked for paraffin wax, there was no inducement to aim at a tar containing undecomposed montan wax, the parent substance of paraffin. Bornstein has shown that distillation at very low temperatures yields tars which differ in their character from those obtained in the above-mentioned commercial processes. Together with my collaborators I have resumed these little noticed investigations, and have developed them in various directions. I shall revert later to the carbonisation of fuel at high temperatures.