312 Prof. P. W. AYood on Destruction of the Fluorescence 



piece of plate glass and breaking off thin scales by tapping 

 the edge with a hammer, striking the blow in a direction 

 nearly parallel to the silvered surface. The silvered scales 

 obtained in this way usually have one edge of razor sharpness, 

 and this edge forms the vanishing line of the photometer. 

 The silvered mirror reflects to the eye the light from the 

 comparison source, which is placed in such a position that 

 the glass sliver cuts across the fluorescent cone at its brightest 

 point. By rotating the graduated nicol prism the intensities 

 may be perfectly matched, the sharp edge of the illuminated 

 mirror vanishing. The intensity of the fluorescence is 

 measured by the square of the cosine of the angle through 

 wmich the nicol has been turned, measured from the position 

 of complete extinction. 



The bulb containing the iodine was in communication with 

 a Gaede pump, a manometer, and a reservoir of the gas 

 under investigation. The bulb was first highly exhausted 

 and the intensity of the fluorescence measured. Air or some 

 other gas was now introduced until the manometer showed a 

 pressure of 1 mm., and the intensity again measured. The 

 pressure of the gas was increased by progressively small 

 steps, the intensity being measured for each pressure. 

 Plotting the results, with the intensities as ordinates and the 

 pressures as abscissae, gives us a curve showing the rate at 

 which the intensity of the fluorescence decreases with in- 

 creasing gas pressure. Experiments were made with air, 

 hydrogen, carbon dioxide, and ether vapour. Several series 

 were made with each gas, and the results were in good 

 agreement. 



The curves are reproduced in Plate III. together with curves 

 obtained with other gases, which will be discussed in the 

 following paper. 



An examination of these curves showed that the hypothesis 

 of free path and saturation point would not represent the 

 facts, and that some other factor must be taken into con- 

 sideration. 



The effectiveness of the gas in destroying the fluorescence 

 appeared to increase with its molecular weight, but was by 

 no means proportional to it. For example, the intensity of 

 the fluorescence was reduced from 45 to 8 5 by 3 mm. of 

 ether vapour, 7 mm. C0 2 , 11*5 mm. air, and 24 mm. of 

 hydrogen. It seems clear from these results that some other 

 property of the gas than its molecular weight must be 

 operative. 



In the case of the fluorescence of anthracene, Elston found 

 that the presence of hydrogen and nitrogen w T as almost with- 



