384 ANIMAL PHYSIOLOGY. 
blood is “ pure blood” in the sense that it contains no carbonic 
anhydride, as in reality it always carries a large percentage of 
this gas. 
Fia. 315.—Diagrammatic illustration of Ludwig’s mercurial gas-pump. A and B are two 
glass globes, connected by strong India-rubber tubes, with two similar glass globes, 
A’ and B’. A is further connected by means of the stop-cock c with the receiver C, 
containing the blood (or other fluid) to be analyzed ; and B, by means of the stop-cock 
d and tube e with the receiver D, for receiving the gases. A and B are also connected 
with each other by means of the stop-cocks f and g, the latter being so arranged that B’ 
also communicates with B’ by the passage g’. .A’ and B’ being full of mercury. and the 
cocks k, f, g and d being open, but ¢ and g’ closed, on raising A’ by means of the pulley p 
the mercury of A’ fills A, driving out the air contained in it into B, and so out through e. 
‘When the mercury has risen above g, f is closed; and gy’ being opened, B’ is in turn raised 
till B is completely filled with mercury. all the air previously in it being driven out 
through e. Upon closing d and lowering B’, the whole of the mercury in B falls into B’, 
and a vacuum consequently is established in B. On closing g’ but opening g, f, and k, and 
lowering A’, a vacuum is similarly established in A and in the junction between 4 and B. 
If the cock c be now opened, the gases of the blood in C escape into the vacuum of 4 and 
B, By raising A’ after the closure of c and opening of d, the gases so set free are driven 
from 4 into B, and by the raising of B’ from B through e into the receiver D, standing 
over mercury. (After Foster.) ae 
The Conditions under which the Gases exist in the Blood.—If a 
fluid, as water, be exposed to a mixture of gases which it can 
