140 THE INTERFEROMETRY OF 



Cooling of gas as resulting from longitudinal radiation might be suggested, 

 but, as it was not encountered in the case of the steam tube, it would not 

 seem to be menacing here. 



Finally, it will be seen from equation (8) that the effect of a leak is to make 

 AAP too small. It will be larger as the vacuum is more perfect. Hence t' 

 should be too large for this reason. A small /' can not be due to a leak. The 

 exhaustion effect, since the gas expands into a vacuum, can not be serious. 

 None of these incidental difficulties seem adequate to account for the large 

 temperature discrepancies consistently obtained. All things considered, it 

 seems to me most probable that the temperature coefficient, as the gas enters 

 the region of red heat more fully, continually decreases, and that this is the 

 real explanation of the low temperature values obtained. 



The apparatus was now taken apart and provided with a fresh jacket. 

 After drying, the cold apparatus again appeared in good condition. The 

 results with the barometer at 75.55 cm. were 



Cold (22) p io s AN Red hot p io 3 A7V' 

 73.0 17.7 73.0 8.0 



17.8 

 17.8 



Unfortunately the glass cracked after the first experiment at red heat. 

 The data for AAT (cold) agree almost exactly with the preceding results. The 

 high temperature would be t' = 383, again enormously too low. Nevertheless, 

 if the values of a. were in question, as the temperature must have been at 

 least 850, this would come out as low as 01 = 0.0015. The misgivings already 

 enumerated apply here as before. As the experiments are very laborious 

 they were abandoned at this point, for it did not seem that further work 

 would materially enhance the result ; nor was it thought necessary to actually 

 measure the high temperatures. 



84. Flames. In the earlier report on the refraction of flames an abnormally 

 low result of ju was obtained for the ignited gases. I have since repeated this 

 work with additional improvements. It appears that it is quite possible to 

 look through the peak of the blue case (symmetrically) without destroying 

 the interference pattern as a whole, though this naturally quivers excessively. 

 The last of the new results showed for the presence (A/"') and (A/) of the flame 

 the micrometer readings : 



N', flame 0.029 0.029 0.029 0.029 0.029 

 N, air .0297 .0295 .0294 .0296 .0296 



Hence the mean difference is 0.00056 cm., or per centimeter of breadth 

 (2.3 cm.), 



dN = 0.00024 cm. 



If the space occupied by flames were vacuum, the difference would have been 

 0.000268 per linear centimeter. Thus AAf' = 0.00002 cm., which lies within 

 the error of observation, but is otherwise quite of the order to be expected 

 for the hot gases in question. 



