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One of the chief difficulties in studying the colours met 
with in plants is that they are often mixtures of quite distinct 
colouring matters. Sometimes these may be easily separated, 
for one may be soluble, and the other insoluble, in such re- 
agents, as water, alcohol, ether, or bisulphide of carbon. The 
fact of being soluble or insoluble is not, however, an invariable 
proof that the colours are essentially different, since the pre- 
sence of some other colourless, insoluble substance, which 
has a strong affinity for the colour, may prevent solution. In 
many cases, however, there is a mixture of colouring matters 
so closely related that anything like a complete separation is 
perhaps impossible ; but, even in such instances, we may be 
able, at all events, to effect a partial separation by agitating 
the alcoholic solution with bisulphide of carbon or the aqueous 
with ether. In this latter case, for example, both colours 
may be soluble in water and in ether, but the relative amount 
of one may be far greater in the solution of water in ether, 
which rises to the surface, than in the solution of ether 
in water, which sinks to the bottom. On evaporating these 
two different solutions, and examining the residue, it is often 
easy torecognise the presence of two different substances, mixed 
in various proportions ; and, with proper care, their general 
properties may be sufficiently well ascertained. Nature also 
herself often assists us in this inquiry, for different plants, or 
the same in different states, may furnish particular colouring 
matters, comparatively pure, or so variably mixed that the 
character of the mixture may be recognised. So much, how- 
ever, depends on the particular circumstances, that it would 
be difficult to give any rules applicable in all cases, and I 
will, therefore, describe what appears requisite when treating 
of special examples. 
As already named, the number of different coloured sub- 
stances met with in plants is very great—probably there are 
hundreds. I shall not attempt to describe individually even 
those found in leaves, but classify them into such groups as 
not only have intimate optical relations, but also are to a 
great extent connected in a similar manner with the develop- 
ment of the leaves. 
1. The chlorophyll group is distinguished by being insolu- 
ble in water, but soluble in alcohol and in bigulphide of 
carbon. ‘There are three or four species, giving well-marked 
spectra, with several narrow, dark -absorption-bands, one or 
more of which occur at the redend. The mixed chlorophyll 
of ordinary green leaves may be obtained in a tolerably satis- 
factory state by heating in alcohol dark-green holly leaves, 
previously well crushed, so as to insure rapid solution, and 
