472 On the Absorption of Light at different Temperatures. 



produced in the position of the absorption-bands of a solution 

 of carmine by mixing it with other coloured solutions, and was 

 thus the first to call attention to phenomena of this nature. 

 The following observations, on analogous phenomena presented 

 by indigo, had been already made by the author at the date of 

 Professor Melde's communication. 



It is well known that indigo gives a spectrum in which a com- 

 paratively narrow red band is followed by an absorption-band, of 

 greater or less breadth according to the concentration of the 

 solution, while after this there comes a bright band which attains 

 its maximum of intensity in the blue, and lastly the violet end 

 of the spectrum again suffers absorption. Now if a small quan- 

 tity of a solution of sulphate of copper is mixed with such a 

 solution of indigo, the red line disappears immediately, and after 

 a short time the second bright band begins to approach the red 

 end of the spectrum, and finally extends in this direction to the 

 extent of about one-eighth of the breadth of the entire spectrum. 

 The indigo solution, however, must not contain any free sulphu- 

 ric acid ; for if only one drop of this acid is added to it the first 

 spectrum reappears. It thus becomes a question whether the 

 change above described does not depend on the formation of a 

 new chemical compound. On adding dichromate of potassium 

 to the indigo instead of sulphate of copper, a much smaller alte- 

 ration takes place. The red band then remains unchanged, and 

 after adding several drops we observe only a displacement of the 

 limits of the green towards the red end, the maximum displace- 

 ment amounting to about one-thirtieth of the total breadth of 

 the spectrum. 



The solutions examined by the author with respect to altera- 

 tions of absorption caused by changes of temperature, were ferric 

 chloride, cupric chloride, cupric sulphate, sulphate of cupram- 

 monium, dichromate of potassium, sesquinitrate of nickel, proto- 

 chloride of cobalt, and dichloride of platinum. In all these an 

 alteration was manifest, namely in all cases an increase of ab- 

 sorbing power with rise of temperature; but this was much 

 greater in the case of chlorides than with the other salts. Chlo- 

 ride of copper, for instance, when employed at the proper degree 

 of concentration, becomes completely opake at the boiling-point. 

 And it is worthy of remark here, that the part of the spectrum 

 which remains longest visible as the temperature is raised, does 

 not coincide exactly with the part which is the last to disappear 

 when the thickness of the stratum is gradually increased ; so 

 that the point of maximum intensity of the spectrum comes at a 

 different place in the heated substance from that which it occu- 

 pies at ordinary temperatures. 



The behaviour of chloride of cobalt is also interesting. At 



