462 THE POPULAR SCIENCE MONTHLY. 



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 win- 

 ter 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 mat- 

 ter. 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 ap- 

 pearing, 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 deter- 

 mination. 



The burning-point ov 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 tem- 

 perature at which the oil becomes unsafe. 



The unreliability of this simple method of testing and the im- 

 portance 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 in- 

 variably 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 



