272 CHEMICAL DISCOVERY AND INVENTION 



owes its colour to the presence of a minute quantity of chromium 

 in a peculiar condition of oxidation, and according to the late 

 Professor Fremy, sapphire contains the same element in a dif- 

 ferent state. In this case the blue colour is more like that which 

 is imparted to fused substances by cobalt, though it has also 

 been attributed to titanium. As to the rest of the precious 

 stones the cause of the characteristic colours is not in all cases 

 known. Thus the emerald and aquamarine are varieties of 

 beryl which probably owe their green colour to the presence of 

 iron in the ferrous state ; that of emerald has, however, been 

 attributed to chromium and even to organic matter. Iron in 

 the ferric state and manganese are capable of producing various 

 shades of yellow, red, or purple, according to the states of com- 

 bination in which they occur, and to one or both of them together 

 the amethyst, the garnet, and other stones probably owe their 

 colour. 



CHAPTER XVIII 



LUMINOSITY OF FLAMES 

 THE INCANDESCENT MANTLE INDUSTRY 



EVERYONE is familiar with the facts that the flame of a candle, 

 an oil lamp, or ordinary coal-gas is more or less luminous, while 

 burning hydrogen, spirit of wine, or gas mixed with air, as in 

 the Bunsen burner, gives so little light that such a flame in 

 broad daylight is scarcely perceptible and in sunlight is actually 

 invisible. What is the cause of this difference ? The attempts 

 to answer this question have occupied experimental chemists for 

 upwards of a hundred years, and though many hypotheses have 

 been put forward it cannot be said even now that the complete 

 solution of the problem has been discovered. 



Sir Humphry Davy was the first to enquire systematically 

 into the source of light in flame, and in 1816 he put forward the 

 opinion that the production and ignition of solid particles within 

 the flame itself is the cause of the light. With regard to the 

 luminosity of coal-gas he attributed the effect to the decom- 

 position of a part of the gas towards the interior of the flame, 

 where the air is in smallest quantity, and the deposition of solid 

 charcoal, which by its ignition and afterwards by its combustion 

 increased to a high degree the intensity of the light. " A few 



