of Edinburgh, Session 1885 - 83 . 
969 
in the case of these coloured salts ? Probably the colourless ones 
have vibrations of high pitch in the ultra-violet; with a higher 
atomic weight and slower vibration they gradually absorb the rays 
of the visible spectrum. The blue rays are first absorbed, and the 
salt appears yellow ; then the green giving orange, and then the 
yellow and orange rays, all also absorbed, giving red. In the 
case of tellerium chloride all are absorbed, and the salt is black. 
Carnelley has therefore demonstrated that the salts of a group have 
molecular vibrations, which are similar, which absorb light and give 
rise to colour sensations , which are similar, changing uniformly, 
however, with increasing atomic weight. If a curve be constructed 
in which the ordinates represent the atomic weights of the positive 
elements, and the abscissae a chromatic scale rising from blue, green, 
&c., to black, we shall obtain a curve, indicating that the colours of 
the compounds are a periodic function of the elements arranged in 
atomic series. This is best seen in the case of the normal iodides. 
Upon the pitch of the vibration depends the colour sensation, as 
every one would admit ; we find in the case of taste a result which 
in every way is the same. 
This has been an induction from the study of the inorganic com- 
pounds, but a whole field of inquiry has been untouched. As yet 
no allusion has been made to the carbon compounds, many of which 
are sapid. 
One of the most important and interesting facts in connection 
with the chemistry of the carbon compounds is that they have what 
may be termed a structure. Thus the formula for common alcohol 
is C 2 H 6 0, but one of the hydrogen atoms is found to be especially 
related to the oxygen, and it is written C 2 H 5 . OH. In this case, 
then, there is one group (compound radical), C 2 H 5 , united with 
another group, OH. Now those groups play the same part that 
elements do in inorganic compounds, and it is necessary to know 
this so-called structural formula,, in order to understand the property 
of an organic substance. Just as we can replace the hydrogen in 
water with potassium and form another substance of different pro- 
perties depending upon the nature of the substance you add, so, in 
like manner, we can replace the C 2 H 5 , or the OH, by other com- 
pound radicles or even elements, and produce a substance whose 
properties w r ill depend upon the nature of the radical added. 
