SCIENCE AND ELECTRIC LIGHTING 479 



It should be observed that it is the vapour which carries the electric 

 current. The current is led up to this metal termination, or electrode, and 

 leaves again from this similar electrode, at the other end of the space. 



Now, there is nothing in that space except molecules of vapour — in this 

 particular lamp the vapour of mercury. When free molecules like this are 

 compelled to carry electricity, they are at first somewhat reluctant to respond, 

 but once they are put into a condition to respond they enter into the spirit 

 of the thing with such excitement and vigour that their extreme activity is 

 none too easy to control. The intensity of the light in this lamp is a measure 

 of the vigour with which the molecules of vapour are responding to the 

 conditions of stimulation which we arrange for them. 



But the fact I want you to note is that the molecules will only respond 

 in their own particular way and they show their peculiarities by the nature 

 of the light they emit. Just as we can only get pansies from pansy seed 

 and grass from grass seed, so does the mercury molecule, the oxygen, or 

 the hydrogen molecule, or that of any other gas or vapour have its own 

 characteristics. One of these characteristics is the peculiar composition 

 of the light it gives out. A casual glance shows that the light from this 

 mercury lamp has a special colour. If instead of letting the different 

 components of the light come to our eyes as they are here, all jumbled 

 together, we put them through a spectrograph, we can separate them and 

 see precisely what are the colours which the mercury molecules are sending 

 out and, when mixed up, give the particular colour which we see. 



This spectrum — or analysed light — from mercury vapour, is intended to 

 represent pictorially the same thing. It will be noted that the colours are 

 the same as in the actual spectrum. 



Here is the spectrum of a lamp in which the molecules of sodium vapour 

 (instead of mercury vapour) are sending out the light. There is practically 

 one colour only — yellow. 



Here is the spectrum of a lamp in which the molecules of neon gas are 

 emitting light. 



This serves to show us what, from the point of view of the lamp maker, 

 are the particular features of lamps in which electricity is made to stimulate 

 the free molecules of vapours or gases. We shall return again to such 

 lamps in a moment. 



But what about this other lamp — the familiar incandescent filament lamp ? 

 This has a filament of tungsten metal which the electricity heats to a high 

 temperature. Tungsten metal is also of course made up of molecules — 

 tungsten molecules. Why do not the molecules of solid tungsten radiate 

 light in the same kind of way as those of mercury which we have just seen ? 

 They clearly do not, as you can see from this spectrum of the tungsten 

 lamp which contains all the colours shaded into one another. Instead of 

 sending out light of a few isolated colours it has light of all colours of the 

 spectrum from violet to red. The spectrogram would be exactly the same 

 as this if the material giving the light were any other incandescent solid or 

 liquid. 



The reason is that the molecules of solid materials are not free, when 

 stimulated, to radiate light individually. They become hot as the solid 

 material which they compose is heated by an electric current ; the elements 

 of which they are made up try to move in the way they would move if they 

 were in free space. But they are constrained and fail to do so. Their 

 efforts are degraded into a confused radiation of all colours. That is why 

 solids, when incandescent, are all very similar in the nature of light they 

 give out. Their light is very similar to that of the sun, which comes from 

 incandescent vapours and gases under extreme conditions of pressure. 



