4 THE PRODUCTION AND USE OF LIGHT 



on fire? This sometimes happens. The fat, however, begins 

 to smoke some time before it gets hot enough to catch on fire. 

 THE EXPLANATION. All kinds of greases, fats, and oils when 

 heated sufficiently hot give off VAPORS in large quantities. This 

 simply means that these fats and oils are changed by heat from 

 liquids into vapors, or gases, exactly as water is changed by heat 

 from its form as liquid water into steam, or water vapor. It is 

 paraffin vapor which burns in the candle flame. 



Exercise 2. The Flashing Point and the Burning Point 



Put a little paraffin in a large iron spoon and slowly 

 heat it until it begins to smoke, VAPORIZE, (Fig. 4). 

 Then test the smoke, vapor, frequently with a burning 

 match to see when it will ignite. At a certain temper- 

 ature of the liquid paraffin in the spoon the vapor 

 becomes dense enough to produce a momentary flash 

 over the surface. The paraffin is then at the FLASH- 

 ING POINT. Continue to heat the paraffin in the 



FIG 4 spoon slowly, testing the vapor as before. When the 



Flashing point va P r becomes dense enough to burn continuously, 

 of paraffin. the paraffin has reached the BURNING POINT. 



DEFINITIONS. The flashing point of paraffin, lard, tallow, or 

 oil is that temperature at which the vapor arising and mixing 

 with air produce a momentary flash when ignited. 



The burning point is that temperature of the substance at 

 which the vapor arising and mixing with air produces a continu- 

 ous flame when once ignited. 



It is now evident that in lighting a candle the burning match 

 must be held against the wick long enough to melt and then 

 to vaporize the paraffin in it in such quantities that the vapor is 

 ignited by the match and burns with a continuous flame. The 

 heat from the candle flame continues to melt and to vaporize 

 the paraffin fast enough to produce a steady flame. 



6. Fresh Air is Necessary. We have seen that the vapor 

 from the heated tallow or paraffin rises and mixes with the air. 

 Is the air really necessary that the vapor may burn? We can 

 answer the question best by trying an experiment. 



