534 Prof. Wood and Mr. Speas : A Photometric 



The intensity curve A is remarkably symmetrical, with its 

 maximum at 20°-25°. As we increase the temperature from 

 — 30° to 0° the intensity increases, probably very nearly in 

 proportion to the increase in pressure, since at these very 

 small densities the intensity of the radiation given out by a 

 molecule is not diminished by the presence of its neighbours. 

 Above 0°, however, the increase is no longer proportional to 

 the increase in the number of molecules, since the vapour 

 begins to destroy its own fluorescence in the same way as- 

 would another gas, such as nitrogen or carbonic acid, only 

 to a very much greater degree. For example, at a pressure 

 of *05 mm. (at 5° C.) the intensity is 33, while at a pressure 

 of 0*1 mm. (at 11° C.) the intensity is but 46, instead of 66^ 

 which is the value which we should expect if there were no 

 interaction between the molecules. Between 17° and 25°, 

 though the number of molecules more than doubles, there is- 

 no increase in the intensity, the increase in the number of 

 radiating molecules being almost exactly compensated by 

 diminution in the intensity of the radiation from each one 

 which results from the presence of its neighbours. Above 25° 

 the reduction of intensity preponderates, and the curve falls 

 rapidly. 



If Ave plot the intensities (ordinates) against pressures 

 (abscissa3) we obtain curve C (dotted), the pressures- 

 (abscissas) being recorded along the curve. This curve gives- 

 us a better idea of the phenomenon than curve A, since in 

 this case the intensities are plotted directly against the- 

 changes in the physical state which influences the radiation. 

 This curve shows us, however, only the change in the 

 intensity of the total radiation emitted by all the molecules 

 with increase of pressure. 



In view of the previous work, in which the diminution in 

 the intensity of the radiation from a constant number of 

 iodine molecules resulting from the presence of foreign 

 molecules was investigated, it is of great interest to determine 

 to what extent the radiation from a given group of iodine 

 molecides is diminished by interpolating other molecules of the 

 same kind, or, in other words, the effect of iodine vapour in 

 reducing its own fluorescence as compared with the effect of 

 other gases. 



This can be done very easily by combining the values 

 shown by curves A and B in the following way. 



At a temperature of 0° the pressure is *03 mm. and the 

 intensity of the fluorescence is 24. We wish now to deter- 

 mine the intensity of the radiation of this same group of 



