Journal, Sept. 1. im ^] P^oyd MicToscopical Society. 123 
Monthly Microscopical 
that tlie red leaves of autumn partially recover tlieir green colour 
when subjected to the fumes of ammonia. The calorific ray of the 
spectrum has probably more to do with the formation of acid than 
the actinic ray. I have noticed in the garden rhubarb that when 
leaves are more exposed to heat and light they become quite red ; 
when tested, the acid reaction of such leaves appears to be increased ; 
the solutions obtained from them are of a deeper red colour, and 
produce the same absorption of the spectrum as the more decided 
red solutions obtained from flowers. Most vegetable blues are 
changed to reds on the addition of an acid, thus showing the ap- 
parent connection between the relative amount of acid in the com- 
position of the colour. It does appear, then, when colour is taken 
alone it affords no conclusive information respecting the chemical 
properties on which it depends. The same tint may be made up 
in an infinite variety of ways from the constituents of white light. 
Starting from green, which may be taken as a state of equili- 
brium between blue and yellow, the colour quickly deepens, and 
passes through blue and violet to red. This change is due to 
oxidation ; while the transition from red to yellow may be regarded 
as a deoxidizing, or reversing process. Modifications are produced 
by the retention of a larger proportion of carbonic acid, by the 
predominance of an acid, an alkaline state, or the presence of nitro- 
gen which accelerates the absorption of oxygen ; and when in contact 
with ammonia, nitrogen may assist in the production of a blue or 
violet colour. 
With regard to the formation of colour, this must be consider- 
ably modified, or accelerated by the presence of oxygen. Plants 
while liberating large quantities of oxygen retain in their juices 
much of the same gaseous matter ; and it is a v/ell-established fact 
that intensity of colour in no small degree depends on the absorp- 
tion of oxygen and the quantity plants are able to store up, — in 
other words, upon their oxidizing power. The oxidizing process 
is greatest during sunlight, when another element also comes into 
play and exercises some influence, that received from the sun's rays 
and stored up as heat, and rendered latent, in the form of cellulose 
or wood, fixed and volatile oils, &c. Latent heat is undoubtedly 
an important accessory in many ways ; as in the formation of the 
saccharine, starchy, and albuminous principles — the ammonia and 
water derived from the soil being necessary ingredients — and in the 
colour-making process, especially when colour is to be stored up 
in the roots of plants. 
In order to observe coloured matters df any kind with any 
degree of certainty they must be examined in a pure spectrum, and 
to do this efi'ectually the material must be so placed that it shall 
intercept a beam of light which forms the spectrum, and when well- 
defined reactions of coloured solutions or materials have once been 
K 2 
