tar yield from bituminous coal. They further give the upper limit to which coal may be heated in rapid working to avoid tar losses by partial decomposition.

Differentiation between Various Primary Tars and other Tars

1 Low temperature and high temperature tars from bituminous coal

The need has been expressed for some chemical criteria by which primary tars can be readily distinguished from ordinary tars. The characteristics given below may not definitely exclude the numerous varieties ranging from useless, solid and pitch-like producer tar to coke-oven tar, vertical retort tar, Mond gas tar and finally to low-temperature tar of high value, and there may be a need for a closer definition of true primary tar later on. The points specified below, however, will suffice as a guard against the crudest intentional sophistication.

(1) General Characteristics:

A good genuine primary tar from bituminous coal should satisfy the following general conclitions:

1. It should be fluid at room temperature, with possi bly slight segregations of paraffin wax.

2. Its specific gravity at 23 degrees should be 0.95 to 1.06.

3. In a thin layer it should be an oil of a golden-red or port-Wine colour.

4. Freshly made it generally smells of hydrogen sulphide or ammonium sulphide; it should never smell of naphthalene.

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