iron, or magnesium, without breaking
down the molecule, as is done in the case of the porphyrins. The actual
binding linkage is undoubtedly between carbon atoms, as indicated in
Willstaetter's formulas for aetiophyllin and aetioporphyrin (see page 109).
The attachment of the magnesium to each one of the four nitrogen atoms in
the skeleton formula assumes the existence of subsidiary valences of 2-4
for magnesium (and of 3-5 for iron), or of possible _oscillating_ valences
similar to those supposed to be exhibited by carbon in its closed-ring
arrangements.

PROPERTIES OF THE CHLOROPHYLLS

The phytyl esters, or natural chlorophylls, are amorphous solids; while the
methylethyl esters (chlorophyllins) and the free acids (phyllins) are
crystalline compounds. All of these compounds are easily soluble in ether
and alcohol, but insoluble in water. The chlorophylls and chlorophyllins
are practically insoluble in petroleum ether and chloroform; but the
monobasic acids (pyrrophyllin and phyllophyllin) and the neutral
aetiophyllin dissolve easily in chloroform.

Solutions of the chlorophylls are fluorescent, being green by transmitted,
and red by reflected light.

Chlorophyll _a_ is a blue-black solid, which gives dark green solutions in
all of its solvents. Chlorophyll _b_ is a dark-green solid, which yields
brilliant green solutions. Solutions in ether of glaucophyllin and of
cyanophyllin are blue; of rhodophyllin, deep violet; of rubiphyllin, light
violet; of erythrophyllin, red; and of pyrrophyllin and phyllophyllin,
bluish-red. Solutions of the porphyrins are all red, the di-basic ones
being usually a bluish-red, and the simpler ones a brilliant red to deep
brownish-red in color.

The several chlorophyll derivatives are further distinguished by
characteristic differences in their absorption spectra. These differences
have been pictured by Willstaetter in his book dealing with the results of
his investigations concerning the chlorophylls, and reproduced in one or
two other texts which treat in detail with the physical-chemical properties
of these pigments, but need not be presented in such detail here.

THE CAROTINOIDS

The characteristic brilliant green of healthy plant tissues is due to the
fact that there are always associated with the dark bluish-green
chlorophylls two (or more) yellow pigments. These are known as the
"carotinoids." This group includes the two b