286 /ngvar Jorgensen and Walter Stiles. 
C. The Yellow Pigments. 
Although the yellow pigments may have physiological import¬ 
ance in carbon-assimilation there is not much to be said in regard 
to their chemistry. They both give non-fluorescent yellow solutions 
stable in alkaline but very easily dissociated in acid media. 
Carotin is identical with the yellow pigment of carrots. It is 
an unsaturated hydrocarbon of the formula C 40 H 5G crystallising 
in rhombohedra with a lustrous blue surface, but appearing red in 
transmitted light. It is easily soluble in chloroform, carbon- 
disulphide and benzene, soluble with difficulty in petrol ether and 
ether, and in even boiling methyl and ethyl alcohol; in the cold it 
is almost insoluble. 
Characteristic of it is its distribution between petrol ether and 
methyl alcohol. If to a solution in petrol ether is added methyl 
alcohol containing a little water, the alcohol layer remains colour¬ 
less. 
It undergoes auto-oxidation. If it stands in air it becomes 
bleached and increases in weight by 35% in dry air and by 41% in 
moist air. With the halogens it forms addition compounds. 
It gives a red solution in carbon-disulphide and a deep blue 
solution in concentrated sulphuric acid. 
Xanthophyll has the formula C 40 H 6G O 2 . The crystals are 
pleochromatic often with a steel blue lustre. In transmitted 
light they are yellow and only red where two or more cross one 
another and in this way are easily distinguishable from those of 
carotin although the colour of the two pigments in solution is very 
similar. The behaviour of xanthophyll with sulphuric acid and 
halogens is the same as that of carotin. It is insoluble in petrol 
ether, the solvent is not even coloured; in methyl alcohol it is 
soluble with difficulty but more easily than carotin It is also 
soluble with difficulty in carbon-disulphide. In ether it is more 
soluble, and is easily soluble in chloroform. Like carotin it under¬ 
goes auto-oxidation and a solution of xanthophyll bleaches in 
presence of air very quickly, much quicker than carotin. 
If a xanthophyll solution in methyl alcohol is mixed with 
petrol ether and a little water added, the greatest part of the 
pigment remains in the methyl alcohol layer. 
Although carotin and xanthophyll give very similar solutions 
it is difficult to compare the colour intensities of the two because 
the colour varies with the solvent and the concentration. The 
carotin is always stronger and in dilute solutions they are not 
comparable because the shade varies. 
