THE STREAMEES IN THE ELECTRIC SPARK. 



79 



small amount is introduced. In the condensed spark without inductance they are 

 usually very bright at the start, but their luminosity dies away much more rapidly 

 than that of the ordinary spark lines. It will be convenient to speak of them as 

 " condensed spark " lines, although it would be premature to suppose that there is 

 any absolute distinction between them and the ordinary spark lines. 



Numbers for the durations do not, of course, represent any very definite property 

 of the lines, and only have a value in the absence of more exact knowledge. It is, 

 however, very striking in most spectra how sharply distinguished in duration the lines 

 of the different classes are from each other. In any particular spectrum the lines of 

 each class seem to have all practically the same duration, and there is a big step in its 

 value to that of the other classes. The durations in many cases vary very little from 

 metal to metal, so that it is possible to arrange the metals in groups and to give ;ui 

 average value of the duration which applies fairly accurately to each group of metals 

 and each class of lines. This is done in the following table : 



The actual differences between the classes are likely to be .somewhat greater than 

 these numbers would imply, both as a result of the whole of the light of the lines not 

 being shown through under-exposure, and also of the fact that the small but constant 

 time during which the electrical discharge occurs and all the lines glow equally is 

 included in them. 



In connection with this evidence it must be observed that the greater duration 

 of lines is not combined with a greater intrinsic brightness at the beginning of the 

 discharge ; on the contrary, the intrinsic brightness of the arc lines at the beginning 

 of the discharge is usually much less than that of the spark lines, although, since 

 their light dies away at a slower rate, they may exceed the latter in brilliancy in the 

 photograph of the stationary spark. The difference between the classes is in reality 

 one of the magnitude of the logarithmic decrement of the atomic vibrations corre- 

 sponding to the lines, the arc lines having a small, and the condensed spark lines a 

 very great, decrement. 



Velocity of the Streamers. 



It is not an easy matter to obtain values of the velocity of the streamers which will 

 serve as a trustworthy basis in the comparison of different sparks, because the shapes 

 of the streamers in the sparks of different metals are often quite different. Compare, 



