Page:Popular Science Monthly Volume 24.djvu/481

Rh of this method depends upon the assumption that a definite relation exists between vapor-tension and flashing - point in all kerosenes. It has, however, been shown in the most conclusive manner, that this is not the case. Four different oils, which all had a flashing-point of 28·5° to 29·5° C, as determined by one of the most trustworthy of the testers before described, were found to give, at 28° C, vapor-tensions of 75, 104, 118, and 168 millimetres (of water); and, at 40° C, tensions of 126, 149, 165, and 201 millimetres. Further, seven different kerosenes gave, when tested by the two methods, the following results:

It thus appears that the results obtained by the measurement of the vapor-tension are quite worthless as indications of the dangerous character of kerosene, and the method must be regarded as far less reliable than even the imperfect ways of testing which have been already discussed.

The uncertainties of the foregoing methods are entirely avoided by a distillation test, which also enables one to decide the quality of the oil as an illuminating material, and thus gives the fullest information in regard to its nature. The oil is separated by the distillation into three fractions: a light oil distilling below 150° C.; illuminating oil coming over between 150° and 270° C.; and a heavy oil which boils above 270° C. The first fractional distillate represents the dangerous constituents, and should not exceed, according to Bielstein, five per cent of the whole. The heavy oil affects the freedom with which the kerosene burns in a lamp, and, in American kerosene, should not form more than fifteen per cent of the oil. The operation must be conducted with care, in a flask provided with a dephlegmator, and the fractions, as well as the original sample, must be weighed. These circumstances are likely to prevent the general adoption of a method which is otherwise so simple and satisfactory, and kerosene will probably be tested in the future, as now, by the determination of its flashing-point.

In 1879 Victor Meyer suggested a principle by which the minimum, or, as he called it, "true or absolute" flashing-point, could be determined. It is to saturate air with oil-vapor at the test-temperature. His method is simply this: A glass cylinder of about 200 c. c. capacity is partly filled with oil, stoppered with a cork through which a