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THE POPULAR SCIENCE MONTHLY. 



selecting one with its manipulation as an arbitrary standard, and adopt- 

 ing conditions in the others which shall give corresponding results. 

 Nor can it be affirmed that all the conditions under which explosions 

 in lamps are liable to occur are provided for in any single instrument 



Fia. 1. The Satbolt Tester. 



of this class. The oil-reservoirs of our lamps differ much in size and 

 shape, and hence have different capacities. Moreover, the quantity of 

 oil, its surface, and the amount of air in the reservoir with which the 

 vapor mingles, are constantly changing while the lamp is in use and 

 the danger greatest. Again, it is not alone in quietly burning lamps 

 that accidents occur. Probably half are due to upsetting or breaking, 

 and the oil, which would have been safe otherwise, gives rise to ex- 

 plosion or flames under these more dangerous circumstances. 



If it is important to test the oil, it certainly is wise to employ, if 

 possible, a test which shall indicate the lowest temperature at which, 

 under any conditions, inflammable vapors can be evolved, and not to 

 trust to a method which merely proves an oil safe under certain arbi- 

 trary conditions. 



Besides these instruments which aim at a direct determination of 

 the temperature at which an oil becomes dangerous, others have been 

 proposed in which the character of the oil is tested in an indirect 

 manner, by finding the elastic force or tension of its vapor at a given 

 temperature. The tension is measured by the height of the column 

 of water which it sustains. By comparing the tension which any oil 

 gives in this apparatus with that of some kerosene which has been se- 

 lected as a standard, the quality of the former is ascertained a higher 

 tension indicating a more dangerous oil. It is plain that the reliabil- 



