514 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
121. A collection of sea-weeds appeared all more or less sensitive, most of them 
highly so. All, or almost all, except the white ones, exhibited in the derived spectrum 
the peculiar red band indicative of chlorophyll and its modifications. The transmitted 
light also exhibited more or less the absorption bands due to this substance, which 
was likewise, in the specimens tried, extracted by alcohol. But the most remarkable 
example of sensibility found in sea-weeds occurs in the case of the red colouring 
matter contained in orangy red, red, pink, and purple sea-weeds. To judge by its 
optical properties, this colouring matter appears to be the same in all cases, but to 
be mixed in different proportions with chlorophyll, or some modification of it, and 
probably other colouring matters, thus giving rise to the various tints seen in such 
sea-weeds. The derived spectrum exhibited by sea-weeds of this kind consists 
mainly of a band of unusual brightness, containing some red, followed by orange 
and yellow. This band fades away gradually at its less refrangible limit, where it is 
separated by a dark interval from the narrow well-defined red band of still lower 
refrangibility due to chlorophyll. At its more refrangible limit, however, it breaks 
off with unusual abruptness. 
122. When the light transmitted through such a sea-weed is subjected to prismatic 
analysis, in addition to one at least of the absorption bands due to chlorophyll, there 
is seen a band obliterating the yellow, another dividing the green from the blue, and 
a third, far less conspicuous, dividing the green into two. The whole of the green 
is absorbed more rapidly than the blue beyond, and not merely than the red, which 
last is the final tint. 
1 23. The red colouring matter is easily extracted by cold water from certain 
kinds of red sea-weed, if fresh gathered ; but when once the plant has been dried, the 
colouring matter cannot be extracted in any way that I know of. It is apparently 
insoluble in alcohol and ether, and is decomposed by boiling. Cold water extracts 
only a trace of it after a long time. 
124. A piece of recently gathered red sea- weed, on being mashed with cold water, 
readily gave out its red colouring matter. When the residue was treated with 
alcohol, the fluid was almost immediately coloured green by chlorophyll, whereas 
this substance is only very slowly and sparingly extraeted by alcohol from dried sea- 
weeds. A dried sea-weed may apparently be assimilated to an intimate mixture of 
gum and resin, which it would be very difficult to dissolve, whether it were attacked 
by water or alcohol. 
125. The solution of the red colouring matter was highly sensitive, exhibiting a 
copious dispersive reflexion of a yellowish orange light. The transmitted light was 
pink or red, according to the thickness through which the light passed. When this 
light was analysed, the same three absorption bands which have been already men- 
tioned were perceived. The analysis of the light transmitted by the fronds of various 
red sea-weeds had rendered it extremely probable that the faint division in the 
