Regular Reflexion of Light by an A hsorbing Gas. 333 



reproduced on Plate VI. fig. 1. The temperatures of the 

 mercury vapour are as follows : — 



Temp. Pressure of Hg vapour. 



a 23° -00168 



b 40 -00574 



c 76 -0750 



d 100 , -276 



e 175 11-00 



The bulb at room temperature (a) appears more or less 

 filled with the resonance radiation. This, as has been shown 

 previously, is due to the fact that the radiation from the 

 molecules which lie in the path of the primary beam excite 

 to resonance the entire mass of vapour in the bulb. At 40° 

 (6) the pressure has increased about 3*4 times, and the 

 radiation comes chiefly from the front part of the bulb. At 

 76° (c) it is confined chiefly to the inner surface of the plate, 

 though a slight haze to the right of the image indicates that 

 some radiation capable of exciting resonance still penetrates 

 to a depth of a millimetre or so. This radiation is without 

 doubt of wave-length slightly greater and slightly less than 

 that of the centre of the exciting line, in other words the 

 edges of the 2536 line. The size of the patch of resonance 

 radiation contracts rapidly as the temperature goes up, owing 

 to the inability of the radiation to spread out and excite 

 secondary resonance. At 175° (e) it has shrunk to the 

 dimensions of the image of the slit thrown upon the bulb by 

 the monochromator. It will be noticed that in case (d) where 

 the density is 16 times as great as at room temperature, there 

 is still a slight broadening of the image, due to the secondary 

 resonance. The spreading is of the order of half a millimetre, 

 which is about what we should expect from the known 

 stopping power of the vapour at room temperature. A further 

 increase of temperature causes a rapid diminution in the 

 intensity of the scattered resonance radiation, the energy of 

 the primary beam passing off as a regularly reflected wave. 

 The intensity is a maximum in the vicinity of 100° (i. e. at a 

 pressure of about 0*3 mm.). At 150° (pressure about *3 mm.) 

 the intensity has decreased to about half its maximum value; 

 at 200° (pressure 18 mm.) to about one quarter, and at 

 250° (pressure 76 mm.) to perhaps one tenth. At 270° 

 there is absolutely no trace of the scattered radiation. The 

 estimates were made from a series taken under similar con- 

 ditions, with a slit somewhat broader, so that a better deter- 

 mination of the relative densities could be made. Equal 



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