of Edinburgh, Session 1885—86. 
973 
Replace the hydrogen by the higher group C 2 H 5 , or Ethyl, to form 
ethylamine, and the shifting continues towards the red. 
With a series of alcohols, such as methylic, ethylic, propylic 
alcohol, we find that, as we pass to the more complex, heavier, hut 
similarly constituted molecule, a shifting of equally characteristic 
bands towards the red end of the spectrum. It is very much (and 
here we may recall the colour experiments of Professor Carnelley) 
as if we took an instrument, say a violin, and kept loading its 
strings. The notes would be heard deeper and deeper in tone, but 
it would be still recognised as having the tone or quality of the 
violin. 
I have mentioned the above instance, though many others might 
have been adduced, to demonstrate the same fact. We have every 
reason then to believe that the radical 
1 -C-O-H 
ii 
0 
and the radical 
II 
I 
-C-O-H 
H 
of the acid and sweet substances are both in constant and characteristic 
vibration, although this vibration does not affect the rays of the 
visible spectrum. Not only so, but we know that this vibration 
will not be altered in character (from the analogy with the ammonia 
radical in methylamine) being merely shifted to a small extent in 
pitch, when its combinations are changed. 
This is, however, the analogy which we have sought. Both in 
the case of sight, hearing, taste, and, as I hope afterwards to prove, 
in the case of smell, we have these analogous phenomena. On the 
side of sensation we have variations in what we may call quality, on 
the objective side we have matter in vibration, — these vibrations 
vary in character, and it is possible to connect the quality of the 
sensation with the character of the vibration. 
It may be urged that, inasmuch as an acid like hydrochloric acid 
produces much the same taste as oxalic acid, there is no such definite 
