Management of Light in Illumination. 123 



heated by the flame of the lamp, will press on the oil 

 below it with an increased force, which will cause a 

 part of it to descend and overflow the burner and run 

 out into the room ; and these accidents frequently hap- 

 pen even without lighting the lamp a second time, 

 and sometimes without its having been lighted at all, 

 merely in consequence of the ordinary changes which 

 take place in the temperature of the air, especially in 

 rooms which front the south, where these occasional 

 variations of temperature are most considerable. 



As people in general are not aware of the danger 

 to which they are exposed, when lamps with fountain 

 reservoirs, partly filled with oil, are left several days 

 hung up in the rooms which they are destined to illu- 

 minate, it may be useful to explain this matter at some 

 length. 



When a lamp with an inverted reservoir has burned 

 for some time, the oil in the reservoir becomes warm, 

 and the air which now occupies the upper part of it is 

 warm likewise ; but, as soon as the lamp is extinguished 

 and begins to cool, the elasticity of the air in the reser- 

 voir begins to be diminished. And, as the pressure of 

 the atmosphere without remains the same, a part of 

 the oil in the burner and in the canal which leads to 

 it is forced back into the reservoir by the pressure of 

 the external air. 



If the quantity of air in the reservoir is considerable, 

 and the cooling process continues, so much of the oil 

 in the burner and in the canal leading to it will be 

 forced to return into the reservoir that its level will at 

 length be so much lowered that the opening of the 

 inverted reservoir (which is at its lower extremity) will 

 cease to be submerged in this oil ; and, as the cooling 



