THE BLOOD. 
487 
as their shape. Dr. Young estimates a globule of human blood 
at j 'q-q of an inch in diameter: and we may as well sit down 
contented with this, as spend our time in searching after other 
evidence, perhaps, after all, less conclusive. 
The colour of the globules is materially aff ected by atmospheric 
air. If a clot of blood contained in a basin be examined, its 
upper surface, wdiich has been exposed to the air, will be found to 
possess a bright scarlet colour, w 7 hile the low ; er or unexposed part 
will appear of a dark Modena purplish hue : only invert the clot, 
however, and in a short time that part which is dark will turn to 
a bright red, while the other (now excluded from the air) will 
change to a dark purple aspect. This change of colour is wholly 
ascribable to the action of the oxygenous part of the air. Expe¬ 
riment has fully proved this fact; and also, that the other ingre¬ 
dient of the atmosphere, viz. azote (as well as the carbonic acid 
gas), has quite the contrary effect upon the blood, converting its 
scarlet hue into purple. 
Notwithstanding the fluctuating and contradictory accounts of 
chemical inquirers into the composition of the blood, there appears 
little reason to doubt the existence of iron in it; and in the red 
globules in particular the presence of the metal has been detected: 
through which metallic agency, we may add, it is that oxygen 
produces the remarkable change above alluded to. For the pre¬ 
sent, let this much suffice. I shall have occasion to amplify much 
on this part of my subject when I come to speak of respiration. 
SERUM 
Is the yellow or straw-coloured fluid which gradually exudes after 
coagulation from the crassamentum. It has a saline taste, and is 
adhesive in its nature, on which account it is found somewhat 
specifically heavier than water. 
Superficial observation and analogical inference have given rise 
to serious error in regard to the proportionate quantities of serum 
and crassamentum in the blood of the horse. For some hours 
after a horse’s blood is coagulated, it exhibits one uniform gela¬ 
tinous mass whose surface is scarcely moistened by serous exuda¬ 
tion ; whereas that of a man in the same interval of time would 
discover the clot actually sw imming in serum. The truth of the 
matter however, is, that so far from there being a comparatively 
less quantity of serum in horses’ blood, there is actually a larger 
proportion; the difference being that it requires a muck longer 
time for its evolution in the graminivorous than in the carnivorous 
animal. 
Take a pint of blood from a man, and place it in a temperature 
of 50°, and in the course of three days it will not only have per- 
