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XVIL.—Observations on the Principle of Vital Affinity, as illustrated by recent 
discoveries in Organic Chemistry. By Witt1amM Putrenry Auison, M.D., 
F.R.S.E., Professor of the Practice of Medicine in the University of Edin- 
burgh. 
(Read, 2d February 1846.) 
Part I. 
THE most important steps in a science are those which lead most directly to 
the establishment of principles or laws peculiar to that science itself, and which 
constitute its claim to be regarded as a distinct branch of human knowledge. It 
has been long acknowledged that such is the character of many of those pheno- 
mena of living bodies which depend on mechanical movements, or changes of po- 
sition in their particles, and therefore that the laws of vital contractions are to 
be regarded as equally elementary and distinctive principles in physiology, as 
the laws of motion or of gravitation in natural philosophy. But a difficulty has 
been long felt, as to whether a similar claim to peculiarity of the principle on 
which they depend, can be urged for the chemical phenomena of living bodies. 
In laying down the first principles of Physiology and of Pathology, I have, 
however, uniformly maintained the existence of a power peculiar to living bodies, 
and to which the term Vital Affinity, as recommended by several authors, may 
be properly applied ;—a power by which “the elements of nutritious matter are 
thrown into the combinations necessary for forming the organic compounds, and 
restrained from entering into other combinations, to which they are prone as soon 
as life is extinct;—a power which supersedes and counteracts ordinary chemi- 
cal affinities in living bodies, as completely as vital contractions counteract gra- 
vitation or the inertia of matter.”—(Outlines of Human Physiology, p. 22.) And 
in delivering lectures on physiology, I always expressed my belief that a time 
would:come, when discoveries in the chemical department of the science,—connect- 
ing the ingesta of living bodies with the nourishment of their different textures, 
and with the nature of the different excretions,—would elucidate the chemical 
changes which are continually going on in them, and are essential to their living 
state, as completely as the discovery of the circulation of the blood illustrated 
many of the conditions of the existence of living animals. It appears to me that 
this anticipation has been more nearly realized by recent chemical observations, 
than professed physiologists have yet admitted ;—that not only the existence of 
the principle of vital affinity has been established, but its limits and mode of 
VOL. XVI. PART II. 2T 
