CH. xxxiv. SPECTRA OF DIFFERENT FLAMES. 337 



Sir David Brewster, and others, had made many valuable 

 experiments upon the colours produced by different burning 

 lights. You know already that it is possible to make 

 coloured flames by burning certain substances. For in- 

 stance, if you put common salt in a spirit-flame, it will burn 

 with a yellow colour, while a substance called nitrate of 

 strontium will give a brilliant red flame, and is used in making 

 red fire for the theatres. Many other metals and earths, 

 however, tinge the flame so slightly that you cannot see the 

 colour, and it is only by passing the light through a slit and 

 examining it by means of a prism that you can detect it 



Light from flames and from white-hot solids and liquids 

 when passed through a prism produces a continuous spec- 

 trum, that is, a coloured band unbroken by any dark lines. 

 A lighted gas jet, a white-hot poker, or a flow of white-hot 

 molten iron, will all give the continuous spectrum No. i, 

 Plate I. But gases or vapours, when heated so as to be- 

 come luminous without burning, do not give a continuous 

 band of colour, they only produce a few bright lines, such 

 as those in Nos. 3 and 4. You can see this by putting a 

 pinch of salt into a candie-flame, and examining it through 

 a small spectroscope. The sodium in the salt will be 

 reduced to a luminous vapour in the flame, and will give 

 the bright line in No. 3 in the table of spectra, standing out 

 from the continuous spectrum given by the candle-flame. 



Now there is a remarkable peculiarity about these bright 

 lines formed by gases or vapours, namely, that they art 

 different for the gas or vapour of every different substance. 

 Thus, if you burn any substance containing sodium, a bright 

 yellow stripe will appear as in No. 3 ; while hydrogen will 

 give one red, one blue, and one violet stripe, as in No. 4. 

 This test is so true and delicate that the hundred and 

 eighty-millionth part of a grain of sodium will gite the 



