30 INORGANIC EVOLUTION. [CHAP, 



However this may be, we now know that many elements present 

 changes at several widely differing stages of heat. The line spectra of 

 elements like sodium, lithium, and others may be obtained by the heat 

 of the flame of a spirit lamp, or an ordinary Bunsen's burner, the sub- 

 stance being introduced into the flame by a clean platinum wire twisted 

 into a loop at the end. 



This temperature has no effect upon iron and similar metals. To 

 get any special spectral indication from them a higher temperature 

 than that of the Bunsen is required ; the blowpipe flame may be resorted 

 to ; in this a stream of air is blown through the centre of a flame of 

 coal gas burning at the end of a cylindrical tube. 



We get in this way what is called a " flame-spectrum," in which 

 flutings and some lines are seen. In order to obtain the complete line- 

 spectra of some of the less volatile metals, like iron and copper, we 

 are driven to use electrical energy and employ the voltaic current, and 

 (for choice) metallic poles, which are so strongly heated by the passage 

 of the current that the vapour of the metal thus experimented on is 

 produced and rendered incandescent. 



We may say generally that no amount of heat-energy will render 

 visible the spectra of gases. These are obtained by enclosing the gases 

 in glass tubes, and illuminating them by means of an electric current. 

 We may go further and say that the ordinary voltaic current used in 

 laboratories is equally inoperative. We must have the induced 

 current, and with different tensions different spectra are produced. 



We have then arrived so far. Heat-energy, which does give us 

 line-spectra in some cases when metals are concerned, fails us in the case 

 of the permanent gases and many metals. A voltaic current gives us 

 spectra when metals are in question, but, like heat-energy, it will not 

 set the particles of the permanent gases vibrating. 



But when both metals and the permanent gases are subjected to the 

 action of a strong induced current, that is, a current of high tension, 

 when an induction coil with Leyden jars and an air break are employed, 

 we get this vibration ; gases now become luminous, a distinct change in 

 the spectra of the metals is observed, a change as well marked, or 

 perhaps better marked, than any of the previous lower temperature 

 changes to which I have already drawn attention. 



When the tension is still further increased, the differences in the 

 spectra are most marked in the case of gases, for the reason that, being 

 enclosed in tubes, they cannot escape from the action of the current ; 

 all the molecules are equally affected. The spectrum is sometimes NOT a 

 mixed one. In the case of the metals the spark is made to pass between 

 two small pointed poles, and the region of most intense action is a very 

 limited one ; we get from the particles outside this region the spectrum 



