i 5 4 THE POPULAR SCIENCE MONTHLY. 



the temperature of the oil in an ordinary glass oil-lamp ranges from 

 76 to 98 F., and in a metal lamp from 76 to 129 F., the lower limits 

 being for rooms heated between 73 and 74 F., and the higher for a 

 temperature of 90 to 92. It is, therefore, evident that an oil giving 

 off explosive gases at less than 100 F. must be dangerous, and even 

 at 110 F. an accident might occur, but only in exceptional circum- 

 stances. 



The oils must, therefore, stand a certain test, called the " flashing 

 test," which consists in heating them, preferably, in a thin metal or 

 glass cup which holds the oil, and is itself placed in another vessel full 

 of cold water, which is gradually heated by a small spirit-lamp. The 

 bulb of a thermometer is kept well immersed beneath the surface of the 

 oil, draughts are to be avoided, and the heat very slowly raised. From 

 time to time, as the flashing-point is approached, the temperature is 

 noted, and a very small flame, as a gas-jet issuing from a glass tube 

 drawn to a fine point, is quickly passed across its surface, taking care 

 not to touch the oil. A faint blue flame will flash across the oil when 

 it reaches a temperature at which explosive gases are given off. Al- 

 though it is generally agreed that the temperature should be very 

 gradually raised, fifteen minutes being allowed for a test, yet Calvert 

 (Chemical Neics, May, 1870) states that an oil which flashed at 90 

 F., after fifteen minutes, showed a flashing-point of 101, when thirty 

 minutes were consumed in making the test. Oil of 100 is not safe 

 absolutely. There is another test called the burning-test, the point at 

 which an oil will take fire and burn ; it is from 10 to 50 F. above the 

 flashing-test (Chandler), and is of little value in determining the safety 

 of an oil, because, as already shown, the addition of one per cent, of 

 naphtha will lower the flashing-test 10 in a good oil, while it would 

 not materially affect the burning-point. From the directions already 

 given for testing oil any one can readily make the test, and in view 

 of the large number of unsafe oils sold it is very important that such 

 tests should be made before using an oil not known to be safe. 



The subject of refining petroleum may be dismissed with a few 

 words more about " cracking " oils. It is the object of the refiner to 

 make as much illuminating oil as possible, and to do this advantage 

 is taken of the fact that, when the vapors of heavy oils are heated 

 above their boiling-points, carbon is deposited, and the condensed 

 hydrocarbons resulting have a less specific gravity. This decompo- 

 sition is technically called " cracking," and it was observed long ago 

 that in distilling the heavier oils lighter hydrocarbons were obtained 

 during the first stages of the operation, even when not wanted. 

 Cracking can be accomplished by distilling the oils under presstire, 

 or, as is the case in the very large stills now employed, by allowing 

 the vapors of the heavier hydrocarbons, on condensing, to flow down 

 again upon the now hotter oil in the still, whereby they are cracked, 

 depositing carbon. By carefully adapting the heat to the changing 



