THE SPECTRUM 



315 



Lastly, we may employ Bunsen's well-known apparatus, 

 in which hydrogen is generated by zinc in diluted acid, 

 saturated with sodium chloride, coal-gas being also passed 

 through the liquid to assist 

 effervescence and carry more 

 spray into the exit pipes. 

 One burner is of slit form, 

 and when the supply of air is 

 adjusted gives a hot and bril- 

 liant sheet of incandescent 

 sodium vapour. The other is 

 cylindrical, and has placed 

 over it a conical mantle which 

 checks combustion, and causes 

 a cooler flame, also coloured 

 with sodium. This flame ap- 

 pears nearly black against the 

 brighter one ; and shading 

 both with a metal case or 

 chimney only open in front, 

 both flames are readily pro- 

 jected on the screen by a 

 single lens. 



180. The Invisible Spec- 

 trum, Only one or two key 

 experiments need be men- 

 tioned here for demonstrating 



the existence of invisible heat-waves beyond the red, and in- 

 visible actinic rays beyond the violet of the spectrum. 



Caloresccncc is best shown by Tyndall's experiment of 

 passing the invisible heat rays through an opaque solution. 

 The safest of his arrangements with the highly inflammable 

 solution of iodine in carbon disulphide, and which need give 

 rise to no uneasiness, is to employ a box or lantern with an 

 open circular aperture in front, and a reflecting mirror 



FIG. 173 



