(1.) Tar-oils should not contain more than a trace of constituents insoluble in xylol. The test on this is performed as follows:—25 grammes (0.88 oz. av.) of oil are mixed with 25 cm.^3 (1.525 cub. in.) of xylol, shaken and filtered. The filter-paper before being used is dried and weighed, and after filtration has taken place it is thoroughly washed with hot xylol. After re-drying the weight should not be increased by more than 0.1 gr. (2.) The water contents should not exceed 1 per cent. The testing of the water contents is made by the well-known xylol method. (3.) The residue of the coke should not exceed 3 per cent. (4.) When performing the boiling analysis, at least 60 per cent. by volume of the oil should be distilled on heating up to 300° C. The boiling and analysis should be carried out according to the rules laid down by the Trust. (German Tar Production Trust on Essen-Ruhr.) (5.) The minimum calorific power must not be less than 8,800 cal. per kg. For oils of less calorific power the purchaser has the right of deducting 2 per cent of the net price of the delivered oil, for each 100 cal. below this minimum. (6.) The flash-point, as determined in an open crucible by Von Holde’s method for lubricating oils, must not be below 65° C. (7.) The oil must be quite fluid at 15° C. The purchaser has not the right to reject oils on the ground that emulsions appear after five minutes’ stirring when the oil is cooled to 8°. Purchasers should be urged to fit their oil-storing tanks and oil-pipes with warming arrangements to redissolve emulsions by the temperature falling below 15° C. (8.) If emulsions have been caused by the cooling of the oils in the tank during transport, the purchaser must redissolve them by means of this apparatus. Insoluble residues may be deducted from the weight of oil supplied. Coal tar oil is the distillate of the tar obtained from gas works, from which all valuable commercial materials such as aniline have been removed. Coal oil tar is also known as creosote oil and anthracene oil, the heat value of which is not quite 16,000 B.T.U. per pound. (19) Residual Oils. Residual oil is the residue left after the lighter oils have been distilled from the petroleum, which before the advent of the Diesel engine were useless. Residual oil which was hardly fluid at ordinary temperatures has been successfully used in the Diesel and semi-Diesel types of engines, by preheating it before admission to the inlet valves. The enormously increased demand for gasoline has resulted in a great increase of the formerly useless residual oil so that it is possible that the demand for gasoline will make the production of the residual great enough so that it can be seriously considered as a fuel. (20) Gasoline.