SOME OF THE WAYS IN WHICH ANIMALS BREATHE. 307 
it is thicker because it is not merely watery,—it has suspended 
in it a number of little bodies-which are called ‘‘ corpuscles.” 
Of these corpuscles some are white, and some are what we call 
red, though in a shallow layer they are rather yellowish in 
colour ; but in mass they are red. The redness of the blood is, 
of course, due to the redness of the red corpuscles ; the redder 
they are the redder the blood. 
Note here, then, that the process of respiration appears to 
have as an accessory phenomenon a change in colour; and 
observe that this colour is dependent for its change on the 
addition or removal of oxygen. The colouring matter of the 
red blood corpuscles, which may exist in two stages of oxidation, 
one a higher and the other a lower, is surely enough the body to 
which we must direct our attention if we want to find out the 
secrets of our respiratory processes. The matter itself is known 
as hemoglobin, and chemical investigation has shown that 
more than 90 per cent. of dried red blood corpuscles consists of 
hemoglobin. This hemoglobin may be separated from the 
blood corpuscles, and, after suitable treatment, appears in the 
form of crystals; the crystals are soluble in water, and it is 
possible therefore to have a solution of hemoglobin. 
In 1862, Prof. Stokes, now President of the Royal Society, 
applied to blood that instrument which has done so much in 
revealing to us the composition of complex bodies ; you know, 
of course, that if we examine solar light by means of the 
spectroscope we find the various bands of colour, red at one end 
and purple at the other, marked by dark bars; and that these 
dark bars, which have been accurately mapped out, reveal the 
presence of certain elements in the light; the so-called D lines 
are, for example, the only two that are seen when pure sodium 
is being burnt before a spectroscope. The investigation of 
colouring matters by Sir David Brewster, Sir John Herschel, 
and other philosophers showed that some of these matters when 
examined by the spectroscope present dark bands or bands of 
absorption of light; these are the so-called absorption-bands. 
Now, Prof. Stokes found that arterial and venous blood gave 
different absorption-bands, and that by adding reagents which 
reduced the blood or took up its oxygen he could convert the 
two bands of any hemoglobin into the single intermediate band 
of reduced or venous hemoglobin. 
