226 SCIENCE PROGRESS 



assumes those colours. Next, the green and blue waves arrive, 

 but since the red and yellow are still there, the poker does not 

 appear, green and blue, but white. Then, at a still higher 

 temperature, we get violet and ultra-violet waves. The spectrum 

 now appears as a regular, uninterrupted band of radiation, 

 of which a small part in the interior is visible as a strip of 

 brilliant colours. A spectrum of this kind is said to be " con- 

 tinuous." It may be obtained from any solid or liquid body 

 whose temperature is sufficiently high. But now let us heat 

 our poker still further, until it is vaporised. To do this we 

 make it one of the poles of an electric arc. If we now examine 

 the spectrum of the flame surrounding the arc, we shall find that 

 it is no longer continuous. It is broken up, as it were, into a 

 large number of lines, of various grades of intensity, with 

 dark spaces between. This means that, instead of having 

 radiation of all wave-lengths, we are getting only the particular 

 wave-lengths represented by the line images of the slit of our 

 spectroscope. This spectrum is called the " flame " spectrum. 

 If we examine the inner core of our arc, instead of the outer 

 flame, we find now that, corresponding to the higher temperature, 

 we obtain a spectrum similar to that of the flame, but containing 

 more lines. This is the " arc " spectrum. If, finally, we pass 

 an electric spark between two pieces of our poker, the radiation 

 we obtain gives yet another spectrum — the so-called ** spark " 

 spectrum. It is similar to that of the arc, but some of the lines 

 in the latter are strengthened while others are weakened, and 

 new lines make their appearance. These new lines, and those 

 which are strengthened in passing from the arc to the spark, 

 are usually called " enhanced " lines. 



Now what is the explanation of this variation of the spectrum 

 with the conditions of production ? In general terms it is quite 

 simple. Radiation arises from processes taking place inside the 

 atom or molecule. A radiating atom may be compared to 

 a peal of bells, capable of giving certain notes and no others. 

 But if those notes are to be heard separately and distinctly, 

 the bells must have free and uninterrupted play. The atoms of a 

 solid or liquid body are not in a state to allow of this. They 

 are always well within one another's sphere of influence, so that 

 the mechanism responsible for the radiation suffers violent 

 disturbances, and instead of the sharp, clear lines character- 

 istic of the free atom, we get a confused medley of radiation 

 of all kinds, " like sweet bells jangled, out of tune, and harsh." 

 The result is a continuous spectrum. But when we vaporise 

 our substance, the conditions are quite different. The atoms 

 and molecules of a gas have a certain '* mean free path," in 

 which they are practically unaffected by their neighbours, 

 and they can therefore emit their characteristic radiations 



