40 



J. G. MACGEEGOE ON THE 



mum absorptiou chauged from wave length 511'4 to 511'6, 512'4 aud 51-t*7 respectively, 

 moved therefore slightly towards the red. 



They made also a series of photometric observations with a solution containing 0001 

 per cent, of salt the results of w^hich are given in the following table : — 



If these results be treated graphically it will be seen that from about wavelength 

 646 to wave length 540 the low temperature curve is the higher of the two ; that from 

 this position downwards the high temperature curve is the higher, but that it and the low 

 temperature curve gradually approach one another after turning away from the line of 

 zero intensity from w^ave length 51*7 9 on. Where the turn occurs cannot be determined 

 exactly ; but if it be noted that between wave lengths 562-'7 and 51'7'9 the low tempera- 

 ture curve has to descend through a distance '736, the high temperature curve only 

 through 620, and that between these wave lengths both have to turn and attain almost 

 the same upward slope, it is obvious that the high temperature curve would seem to 

 have its turning point at a higher wave length than the other. In other words the etieet 

 of increase of temperature seems to be to increase somewhat the intensity of the violet- 

 ward penumbra, while leaving the position of its boundary iinchanged, to move the point 

 of maximum absorption very slightly towards the red, to increase somewhat the inten- 

 sity of that portion of the red-ward penumbra which is near the point of maximum 

 absorptiou, aud to diminish the intensity of the remaining portion. 



Finally Gr. aud H. Kriiss quote experiments by Dr. Miiller ' wnth a 1 per cent, solution 

 of this salt in which he found that at 20' C. the region of maximum absorption was 

 bounded by w^ave lengths 516.9 aud 509.8 and the fraction of the incident light absorbed 

 was 052, while at 60-10' C. the region 518 — 5109 showed a maximum absorption 

 amounting to 059. 



Thus the results of all these experiments with the potassium salt of eosin are in close 

 agreement with the deductions from the working hypotheses made above. 



Knoblauch also examined the etfect of increase of concentration on solutions of three 



' Kolorimetrie,' pp. 278 and 280. 



