On the Absorption of Light at different Temperatures. 471 



He has also discussed Gerhardt's misuse of the word acid. His 

 conclusion that the word had better be given up, would be quite 

 worthy of serious consideration if the words were only used in 

 those improper senses. But the words acid and base really mean 

 something not only true, but of fundamental importance, which 

 we are constantly obliged to consider and speak of in chemistry ; 

 and I am quite sure that it would be utterly beyond my power 

 to take from them their established meaning, even if I wished 

 to do so. There is at present a considerable amount of incon- 

 sistency in the prevailing use of these, as of most other scientific 

 terms; and Mr. Foster's interesting paper affords further argu- 

 ments than those which I had given in favour of abandoning as 

 speedily as practicable the misuse of the terms which has crept into 

 partial use through popular disregard of water in hydric sul- 

 phate, and which Gerhardt unsuccessfully endeavoured to incor- 

 porate with scientific language. 



I have not discussed the proposal to call both acids and bases 

 oxides, because it has not as yet received sufficient development 

 to enable me to form any opinion upon it beyond the obvious 

 objections which present themselves at first sight to so grave a 

 change. Thus MnO, Mn 2 O 3 , MnO 2 , MnO 3 , Mn 2 O 7 are at pre- 

 sent conveniently distinguished by names, calling the last two 

 acids, and the first two oxides; and so also CO and CO 2 are 

 very conveniently distinguished by the words oxide and acid. 

 Another circumstance which would alone have been sufficient to 

 prevent my offering any opinion on this proposal is the fact above 

 explained, that it is founded on Mr. Foster's opinion that the 

 word acid is not clear and is unworthy of being retained. 



University College, London, 

 May 16, 1865. 



LXIV. On the Absorption of Light at different Temperatures, 

 By M. Fetjssner*. 



THE prismatic examination of light which has passed through 

 absorbent media has been continually acquiring greater im- 

 portance ever since Stokes called attention to its practical utility. 

 In particular it is interesting to examine the alterations in ab- 

 sorption which take place on mixing two absorbing substances 

 which exert no chemical action upon each other, and the altera- 

 tions caused by changes of temperature. 



Professor Melde of Marburg has described f the modifications 



* From the Monatsber. d. koniyl. preuss. Akad. d. Wissensch. z, Berlin, 

 March 30, 1865, p. 144. 



f PoggendorfF's Annalen, vol. cxxiv. p. 91. 



