ON THE EXTERNAL CAUSES OF DISEASE. 
367 
afterwards saturated. Carbonic acid forms an exception, for on 
the removal of this serial acid, the blood resumes its bright arte¬ 
rial colour. 
Alkalies, like acids, darken the colour of the blood, but salts 
produce a bright and vermillion colour, when added to the 
colouring matter of the blood. The alkaline carbonates require 
particular notice. The acid is so feebly held by the base, that 
in some respects they conduct themselves as alkalies ; and in 
particular will restore the blue colour to the reddened litmus. 
Dr. Stevens believes that this circumstance has led some che¬ 
mists of great celebrity to admit the presence of free alkali in 
the blood, whilst he takes an opposite view, and believes that in 
venous blood there is a superabundance of free carbonic acid, 
which, however, is soon removed by exposure to air. This opinion 
seems to be confirmed bv the fact, that if water holding cai bo- 
nate of soda and carbonic acid in solution be added to the blood, 
a deep venous hue is produced. After a short time, by exposure 
to the air, the carbonic acid is removed by the attraction already 
noticed, and the blood is reddened by the carbonate of soda, the 
influence of which is no longer controlled by the redundant acid. 
The Doctor, it appears, does not regard the air as passively 
receiving the carbonic acid as it is liberated from the blood, 
which had not only held it in solution, but actually imbibed it; 
he considers that it is actively removed by the attraction exist¬ 
ing between oxygen and carbonic acid, which overcomes the 
weaker attraction by which the acid was united with the blood. 
When the blood has lost its carbonic acid, it presents the bright 
vermilion tint which naturally belongs to its colouring matter, 
and salts when combined. When the venous blood gives up its 
carbonic acid, it receives in exchange a portion of the inspired 
air, which is chiefly at the expense of the oxygen. 
From this it will be perceived that the Doctor differs, in some 
respects, from the theory we have formed respecting the atmo¬ 
spheric air being received into the circulating current;—he sup¬ 
posing that it is only oxygen which is received by the blood in 
the lungs, and only carbonic acid which is exhaled. But we 
have, we hope, already satisfactorily proved, that nitrogen is taken 
into the circulation as well as oxygen. The following experi¬ 
ment of Sir H. Davy will confirm those already given. Sir Hum¬ 
phry respired 18 cubic inches of common air during one minute. 
Before the experiment this air was found to contain 9.3, and 
after it only 4.1 cubic inches of azote, shewing that 5.2 cubic 
inches had disappeared. This experiment was variously repeated 
with the same result, and its accuracy has been subsequently 
confirmed by Dr. Henderson and others. 
