38 



J. G. MACGEEGOE ON THE 



483 and 4*71"4 the curve of the stronger solution is above the other. A.t 4tl"4 it crosses 

 the curve of the weaker solution. And consequently if both curves be produced so as to 

 maintain their general trend, until they cut the line of zero intensity, the curve for the 

 stronger solution will cut it at a point nearer the red than that for the weaker solution. 

 In other words, increase of concentration seems to extend the region of complete absorp- 

 tion towards the red, while it diminishes the absorption in the penumbra up to wave 

 length 483. 



"With regard to the effect produced by elevation of temperature on the absorption of 

 solutions of this salt, G-. and H. Kriiss give' the following observations on a 1 per cent, 

 solution : — 



But these observations show only that elevation of temperature slightly increases the 

 intensity of the absorption at two regions of the penumbra. Melde," however, found that 

 in solutions of this salt, elevation of temperature is accompanied by an extension of the 

 absorption region towards the red. He gives no quantitative details. 



Thus the available observations on solutions of Potassium nionochromate also, so far 

 as they go, seem to support the conclusions reached aboA'e. 



The above results are, for the reasons already stated, by no means conclusive, even 

 with regard to solutions of the four salts to which they refer ; and even if they were, no 

 general conclusion could be drawn from so few instances. But they are all in agreement 

 with the deductions made from the working hypotheses referred to above, and they there- 

 fore shew it to be desirable that a more complete investigation of this subject should be 

 made. 



Note added November 'llh, 1891. — Since the above was written a paper has been pub- 

 lished by O. Knoblauch ' on the effect of increase of concentration on the absorption 

 spectra of dilute solutions. His results enable us to apply additional tests to the conclu- 

 sions reached above. 



Of the solutions already referred to, Knoblauch examined only those of Potassium 

 monochromate. He found that the absorption of a solution of this salt, containing 400 

 grm. of salt per cu. dm., began at wave length 490 (millionths of a millimetre) and be- 

 came complete at wave length 482, whereas the absorption of a solution, containing only 

 0023 grm. of salt per cu. dm., began at wave length 510 and became complete at wave 

 length 496. Increase of concentration therefore was accompanied by extension of the 

 luminous portion of the spectrum towards the violet. The thicknesses of his solutions 



1 • Kolorimetrie,' p. 278. 



'' ' Pogg. Ann.,' Bd. cxxvi (18U5), p. 264. 



■"Pogg. Ann.,' Bd. xliii (1891), p. 738. 



