The distance of the oil, or rather of the foam into which the surface is broken by the air-current, from the top of the cylinder, however, makes a considerable difference in the results—the flashing-point falling as this distance is decreased, until at about 5 to 6 c. m. it reaches a minimum.
These considerations lead to the following statements and directions for the use of this method:
1. The oil-cylinder should have a diameter of 2·5 to 4 c. m. It may be of any convenient length, provided it holds, when filled, for the test not less than 50 c. c. of oil. With a diameter of 2·5 c. m., the length should be at least 16 c. m.; with a diameter of 3 cm., the least length should be 13 c. m. A good tester may be made from the chimney of a student-lamp, by cutting off the lower part, a little above the contraction. (Glass is easily cut by filing a deep notch at one point, and letting a little gas-flame play slowly back and forth across it in the line of the proposed section, until a crack springs quite through the glass; this crack can then be led in any desired direction by keeping the little flame just ahead of it on the glass.) The whole chimney may also serve as an oil-cylinder by corking the large end. The irregularity of shape at the bottom does not affect the results; but the length makes it rather inconvenient by requiring a correspondingly deep water-bath.
2. The cylinder is filled with oil to a point such that, when the air-current is running, the top of the foam is 4 or 6 c. m. below the mouth.
3. The oil is heated by means of a water-bath, into which the cylinder is plunged to the level of the oil. The temperature of the oil should not rise faster than two degrees a minute.
4. Air is forced through the oil with such velocity that about (and not less than) 1 c. m. foam is maintained on the surface, and a flash-jet brought to the mouth of the cylinder every half degree, or oftener in the vicinity of the flashing-point. The approach of the flashing-point is announced by the appearance of a faint blue halo of burning vapor around the flash-jet; this finally detaches itself and runs down to the surface of the oil, and the reading of the thermometer at this instant gives a trial flashing-point, which may be a little too high if the air-current has been running too long, or not long enough.
The test is now repeated with a fresh sample of the oil, and the air-current started in full strength not less than one nor more than three or four minutes before the flash occurs. It is a good plan, however, to let a very slow current of air bubble through the oil from the time that the tester is put in the water-bath, so as to secure regularity in the heating of the oil.
A very good flash-jet is a little gas-flame from the tip of a blowpipe, or glass tube drawn out to a point.
The advantages of this method are: