212 
Proceedings of Royal Society of Edinburgh. [may 2 , 
Group VII. chlorine, bromine, and iodine have very similar smells, 
and so have the acids they form with hydrogen, and their com- 
pounds with methyl, ethyl, ethylene, &c. Although similar in all 
cases, yet they are not the same. One can distinguish the odour of 
iodine from that of bromine or chlorine. It may be described as 
having more flavour, and not so chlorous. Bromine is like them both, 
having an odour intermediate between the two in quality. From a 
study of the above substances, this is seen to hold good in all cases, 
the odour uniformly changing, often to a slight degree, as we pass 
from the lowest to the highest member of a group. A very marked 
change is seen in the formyle compounds of Group VII. Chloro- 
form has a fragrant and characteristic smell. So has bromoform, 
but it has something else in addition, which is recognised as being 
the odour of iodoform. Bromoform thus connects chloroform and 
iodoform, these latter substances being very unlike one another. 
There may then be so little difference in the odours of com- 
pounds of the same group of elements that they may with difficulty 
be distinguished. On the other hand, the differences may be great, 
there being intermediate sensations produced by intermediate mem- 
bers of the group. One is forcibly reminded of the changes in 
sensation experienced in allowing the eye to traverse the spectrum 
from one end towards the other. In a drawn-out spectrum, only a 
part of which is visible, one passes, say, from orange into yellow, 
and these colours are recognised as being different, and at the same 
time alike. In a shorter spectrum the eye may pass from the 
yellow into the red. The two sensations are quite different, but 
the orange is seen to connect them. The importance of the 
above statements — and they may be verified in the case of the 
few odorous compounds in Group V. — will become apparent when 
they are placed in juxtaposition with identical facts mentioned in 
the previous paper on taste, and some recent observations of Pro- 
fessor Carnelley. 
In those groups of elements whose compounds are sapid, we find 
that the same change in taste sensation is apparent as we pass from 
lower to higher members of the group. Let us take as an example 
Group VII., which furnishes us both with sapid and odorous 
bodies 
