494 EXPERIMENTS ON LIGHT-SPECTRA. 



images of the flame as there are different colours 

 emitted by the flame. 



The vapour of calcium, a metal contained in lime, 

 colours the hydrogen flame orange. Calcium chloride 

 is best adapted for our experiment because it is, of all 

 calcium compounds, the most volatile; it may easily be 

 obtained in a soluble form by dropping slowly a little 

 hydrochloric acid diluted with water upon a small 

 quantity of scraped chalk in a watchglass. The orange 

 colour of the calcium flame is, however, not simple ; it is 

 a mixture of orange and green, as may be easily seen 

 when the flame is viewed through the prism; a green 

 and an orange-coloured image appear by the side of 

 one another. 



When a mixture of some lithium salt, a little 

 common salt, and a few drops of the calcium chloride 

 solution, is made with a splinter of wood or a glass rod, 

 and introduced into the flame, the latter appears to the 

 naked eye like the flame produced by the calcium 

 chloride when alone in the flame. This appearance of 

 the flame is shown on the right side of fig. II. (see the 

 coloured Frontispiece) ; on the left side are represented 

 the four separate images seen when the flame is ob- 

 served through the prism. The two calcium flames are 

 smaller than the two others, because the compounds of 

 calcium are .less volatile than those of sodium and 

 lithium, and are really vaporised only in the inner part 

 of the hydrogen flame. 



These experiments are best performed in the evening, or at any 

 rate in a darkened room, for optical phenomena are only well seen 

 when all stray light is excluded. Under any circumstances a dark 

 cloth or blackened sheet of cardboard should be placed a few deci- 

 metres behind the flame, so that the latter may be seen on a dark 



