COAGULATION OF THE BLOOD. 
455 
management, a fresh clot may be redissolved by means of 
ammonia, and that after the escape of the ammonia it will 
again coagulate, and afterwards contract in the usual manner, 
though more feebly. Next, he finds that ammonia is always 
to be obtained from the halitus of freshly drawn blood, and 
although the alkalinity of the blood through soda renders the 
ammonia excessively prone to escape, so that a good deal is 
necessarily lost from unavoidable exposure to air, yet he has 
succeeded in collecting about a third by weight of the smallest 
amount which he has found sufficient to keep the blood per- 
manently fluid outside the body. Lastly, he has observed 
that all those circumstances which are known to promote the 
coagulation of blood outside the bodv, such as an elevated 
temperature, free admixture with air, a vacuum, &c., also 
hasten the process in blood mixed with ammonia, or, in other 
words, favour the escape of the volatile alkali; while, on the 
other hand, those things which check coagulation, such as 
cold and occlusion from air, prevent or retard the evolution 
of the gas. To the latter class he has added the remarkable 
fact, that blood remains fluid for many hours under a high 
mercurial pressure, but coagulates when relieved from it. I 
confess that, although I was by no means prepossessed in 
favour of this theory, these facts appear to me to prove 
irresistibly that the cause of the fluidity of blood, after it has 
been drawn from the body, is a minute portion of free am- 
monia holding the fibrine chemically in solution, and that 
the coagulation of such blood is the result of the escape of 
the alkali. The only point on which the evidence appeared 
deficient was the effect of occlusion from air in tubes of dead 
matter, and this defect I endeavoured to supply by the ex- 
periment which I mentioned at last meeting of the Society, 
by which I succeeded in keeping the blood of a sheep fluid 
for three hours within a vulcanized Indian rubber tube, the 
blood coagulating in about two minutes when let out just as 
if freshly drawn from the veins of the animal.* Hence it 
* This experiment was performed in the following manner : One of the 
jugular veins of a sheep having been exposed, it was emptied of blood by 
passing the finger along it while pressure was applied by an assistant at its 
anterior part. The vessel was then opened at two places about three inches 
distant from each other, and into each opening was tied one end of a piece 
of vulcanized India rubber tube, a quarter of an inch in diameter, and about 
eighteen inches long, filled with water, to prevent the introduction of air 
into the circulation. The pressure was removed from the upper part of the 
vein, so as to allow the blood to flow through the tube. It was now 
easy to ascertain, by observing the collapse of the lower part of the vein, 
when a part of the tube was momentarily obstructed by pressure, that the 
circulation was going on freely through the new channel. This having been 
determined, ligatures of waxed string were tied as tightly as possible round 
the tube, at intervals of about two inches, beginning at the end next the 
