366 Messrs. Trowbridge and Richards on the Temperature 



and then to the oscillatory. Since the spectra obtained were 

 widely different, the conclusion, at least in the case of the 

 mercury, is that the structure is complex ; for mercury 

 molecules are monatomic. This conclusion is reinforced by 

 many other facts known about the continuous discharge. 

 Hence the existence of two argon spectra does not give any 

 reason for disbelieving the evidence of specific heat with 

 regard to the monatomic nature of argon. 



From the point of view of a mechanical conception of the 

 causes producing the two spectra of a gas, it is easy to imagine 

 that when the atoms are bound together in the polarized con- 

 dition, the electricity by a succession of readjustments may 

 travel step by step from one end of the tube to the other, at 

 a comparatively low temperature, and so cause quite a 

 different set of electromagnetic vibrations from those depend- 

 ing upon the breaking down of this polarized system. The 

 evidence that the second spectrum given by the oscillatory 

 discharge is due either to the act of separating the atoms 

 from one another, or to the passage of the electricity through 

 the atoms already set free by the heat, has been given above. 

 Hittorf's experiments, in which he was able to send a very 

 strong continuous current through a gas without the pro- 

 duction of light, would seem to indicate that the light is due 

 to energy involved at the moment of dissociation, but the 

 spectra of the solar prominences lead to the opposite inference, 

 and conclusive evidence upon this point is wanting. The 

 dissimilar behaviour of different gases is easily accounted for 

 by considering the two causes which are supposed to resist 

 the dissociation : in the first place, the " polarized" condition 

 of the molecules, and in the next place, the chemical affinity 

 of the atoms for one another. This last force is usually admitted 

 to be greater in the case of nitrogen than in that of hydrogen, 

 hence the difficulty, the irregularity and the abruptness of 

 the transition in the former case. One should expect that a 

 monatomic gas, like argon, where the polarization alone pre- 

 vents the passage of the current, would be easier to change in 

 this respect, as indeed it is. The fact that the second spectrum 

 of mercury is not yery easily obtained militates against this 

 explanation, however. 



Assuming, then, that the red spectrum of hydrogen is due 

 to the sudden occurrence of the reaction 



H 2 = H + H, 



it is very interesting to note that our results agree with the 

 necessary deductions from the law of mass action as applied 



