220 
Group C. On this account the same kind of erythrophyll 
may, for example, be changed by ammonia in one case to 
blue and in another to green, and the only certain method 
to distinguish the different species is to observe the position 
of any well-marked, narrow, absorption-bands, or, in case none 
are developed by reagents, to observe the limit of the general 
broad absorption towards the red end of the spectrum, which 
is almost, or entirely, free from the effects of any such yellow 
colour. In all cases, when it is desirable to identify or dis- 
tinguish colours in this manner, their spectra should be com- 
pared together side by side, and care used to have the solu- 
_ tions of equal depth of colour, and so far diluted that the 
absorption is dark, but not black, or else it is impossible to 
know that they are equally dark. For this purpose the most 
useful spectra are those of the alcoholic solution treated 
with ammonia, of the aqueous solution treated with hy- 
drochloric acid, or of the solution in sulphuric acid diluted 
with two or three times its volume of water, so as to avoid 
any charring effect. Sometimes this solution is so turbid 
that it must be filtered, but it gives an excellent spectrum, 
and also enables us readily to ascertain a very important fact 
connected with these colours. In my paper on some technical 
applications of the spectrum-microscope, I pointed out that, 
when the colouring matter of fresh grapes is slightly oxidized, 
the absorption is lowered towards the red end of the spec- 
trum, and that it'then corresponds with the colouring matter 
of new red wines. By following out this inquiry I have been 
led to conclude that certain substances exist naturally in such 
a state that they are merely decolorized by further oxidization, 
whereas others are first changed into another colour of the 
former kind. On adding by degrees a little hypochlorite of 
soda to a solution of the colouring matter in the diluted sul- 
phuric acid, until it begins to turn paler, and then com- 
paring side by side the spectrum of this with that of the 
unaltered, both carefully diluted with the sulphuric acid and 
water to such a degree that it is easy to see that the intensity 
of absorption is exactly the same in both, it will be found 
that, in the case of some kinds of erythrophyll, the general 
colour has been changed from orange-red to pink, and the 
absorption, instead of extending over the green, also extends 
over the yellow; whereas, in the case of other kinds of ery- 
throphyll, it will be found that no such alteration has been 
produced. The extent to which the absorption is thus 
lowered is tolerably definite, and about equal to the distances 
between the solar lines 6 ands. It is important to note this, 
since, if we had an artificial mixture of the two different 
