in a burning lamp is some 16° Fahr. (9° C). Before the lamp is lighted the oil in it will in most cases have the temperature of the air about it. Our rooms in summer often have a temperature of 90° Fahr., and reach 100° Fahr. in a few exceptional days, while in winter the oil assumes even a higher temperature than this when the lamp is placed—as it often is—near a stove or an open fire.
Hence, it is plain that the lowest temperature at which an oil may evolve inflammable vapors and be considered safe must be put at 116° Fahr., or better still at 120° Fahr.
What, now, are the means for determining the temperature at which these vapors appear, and thus for deciding upon the safety or danger of an oil? It seems at first thought a simple and certain matter. Put a little oil in a cup and suspend a thermometer in it; warm it slowly, and, as the temperature rises from degree to degree, pass a lighted match just above its surface. Presently the match will cause a tiny explosion. This indicates that the dangerous vapors are appearing, and the thermometer now gives the so-called flashing-point of this oil. Go on heating and testing as before, and at last the oil will take fire and continue burning by itself. The mercury is now at the burning-point. But repeat the experiment with fresh samples of the same oil, and you will find that a trifling variation in the conditions will alter the flashing-point to a wonderful extent. The quantity of oil used for the test, the rate of heating, and the range of temperature through which the oil is heated, the distance above the surface at which the match passes—each and all have a marked influence on the determination.
The burning-point—or fire-test, as it is often misleadingly called—is of little value; for not only does it always lie above the flashing-point—which is the real danger-point—but it bears no simple relation to the latter, so that its determination gives really no clew to the temperature at which the oil becomes unsafe.
The unreliability of this simple method of testing and the importance of the problem have called forth numerous suggestions for improvement. Within the last fifteen years no fewer than twenty-five different instruments have been proposed, presenting as many more or less widely modified forms of the simple cup-tester indicated above. The most essential variations are (1) in the size and form of the oil-holder or cup, which in some apparatus is open, in others partly or wholly closed; (2) in the dimensions of the water-bath—which is invariably employed in all as the best means for communicating a slow and uniform increase of temperature to the oil; (3) in the means used for igniting the vapor—a burning match, waxed thread, small gas-jet, electric spark, or little oil-lamp standing on the cover of the oil-cup being the chief devices for this purpose.
But, notwithstanding all the ingenuity displayed, and the elaborate and costly apparatus to which it has in some instances given birth, we