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Page:Popular Science Monthly Volume 24.djvu/479

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find Engler and Haass,[1] at the close of a careful investigation into the reliability of petroleum-testers, in which all the more promising methods were laboriously examined and compared, laying down these general principles, which are to be observed in the construction and use of this class of testers:

1. The quantity of oil must be the same in all experiments.—In the Saybolt tester, for instance, which was adopted in 1879 by the New York Produce Exchange (chiefly, however, for the purpose of determining the burning-point), variations of one millimetre, or about one twenty-fifth of an inch, in the height of the oil, cause differences of some degrees in the flashing-point.

2. The oil must be heated slowly and uniformly.

3. The temperature of the oil at the beginning of the test must be at least 18° Fahr. (10° C.) below its flashing-point (which is approximately determined by a preliminary test). Hence, a low-grade oil, which flashes not far from the air temperature, must be cooled down before an accurate determination can be made.

4. The size and intensity of the flame or spark used to produce the flash must remain unchanged in all tests. Increase in size or intensity lowers the flashing-point.

5. The distance of the flash-flame or spark from the surface of the oil must be the same in all tests. The flashing-point is lowered by decreasing this distance. Care must be taken that this distance is not so small that a local evolution of vapor from the surface occurs.

6. The time during which the flame or spark acts must be reduced to a minimum, increase in the time causing a sensible lowering of the flashing-point.

7. On account of the practical purpose for which the tests are made, the conditions under which the vapor is formed in the tester should correspond as closely as possible to those which determine its formation and explosion in lamps, etc.

Comment upon methods which depend for trustworthy results upon such a formidable array of conditions is hardly necessary; the best apparatus must be electrical and costly, and even then unreliable except in the hands of an expert. We are not surprised to find Mr. A. H. Elliott, in his report of a similar investigation ordered by the New York State Board of Health, giving as his general conclusion: "Of all the apparatus examined, not one can be called perfectly satisfactory. . . . Of the electric testers it may be stated, that any advantage obtained from the use of electricity is more than overcome by the trouble necessary to maintain the galvanic battery and induction-coil." But, even if the performance of some of these instruments is such as to yield concordant results, when all the precautions are carefully heeded, these results can have only a relative significance, and agreement of different testers can only be secured by

  1. "Zeitschrift für anal. Chem.," xx, 1.