120 MODERN SCIENCE READER 



of fresh coal may not extend right down to the grate sur- 

 face and the fuel next the grate will burn with the incom- 

 ing air at the same time as the gas from the green coal 

 burns on the surface. If the fuel bed is thin, the carbon 

 dioxide first produced on the grate comes to the surface as 

 dioxide, and hinders the combustion of the volatile matter. 

 If the fuel be thick the dioxide may be converted into 

 monoxide in its upward passage through the fuel, and this 

 will again hinder the combustion of the volatiles. The 

 final gaseous mixture above the fuel will be very complex, 

 and usually it will be by no means very hot. Experience 

 tells, as explained by Mr. Swinburne, that this mixed mass 

 ought to be kept hot in a non-absorbent furnace until 

 combustion is complete. 



What now deserves attention is a simple means of 

 examination of a fire with the object of ascertaining to 

 what degree combustion has attained. This is blue glass 

 of a deep tint. Blue glass will not permit the passage of 

 light of a wave length greater than blue. It is because it 

 will not permit this that it is blue. High-temperature 

 radiation has the shortest wave length. Violet light has 

 double the number of waves per inch that represent red 

 light, and red light has millions of times the waves per inch 

 as sound notes. Sound would become visible to a man mov- 

 ing fast enough toward its vibratory origin. Low-tempera- 

 ture flame is red, orange, yellow; blue is hot; violet is so 

 potent that it brings about various chemical reactions, as in 

 photography. A red-hot brick seen through blue glass be- 

 comes drab, and gives no illumination. A brilliantly incan- 

 descent brick lined furnace seen through blue glass appears 

 of a light French gray, and is of illuminating quality. 



Now, if a dull flaming fire be observed, such as is 

 obtained if badly mixed gases rise directly upward from 

 the fire to pass among cold tubes, there will be seen through 

 blue glass no illumination above the fire beyond about six 

 inches. The flames are resolved into dark streams of gas; 

 no light comes from them. But if the interior of a furnace 



