REVERSED AND NON-REVERSED SPECTRA. 141 



85. Conclusion. Though the experiments made are of a tentative charac- 

 ter, the inference seems warranted that, so far as my work goes, the tem- 

 perature coefficient a. of air at low temperature is identical with the coeffi- 

 cient of expansion of gases. At high temperatures the value of a seems to 

 decrease rapidly, in proportion as the gas is more highly ionized at red heat. 



It has occurred to me that such ionization might load the gas in relation 

 to the light-wave passing through it, and that the observed excess of index 

 of refraction over the value anticipated at high temperatures might be 

 explained in this way. But air ionized by the X-rays shows no such effect. 

 Neither does the refraction of flames at high temperatures, so far as can be 

 made out, show a large value of the refractive index of the ignited gases. 



It is difficult to see how the experiment at red heat can be improved, unless 

 a quartz tube is made for the purpose. But even here the difficulty of obtain- 

 ing adequately plane parallel ends and a tube of sufficient breadth is formid- 

 able. The attempt to grind in reentrant glass cylinder-like stoppers at the 

 end of the tube was thought of, but did not succeed. 



