represented as constantly occurring in the tis- 
sues, has been unbalanced by the reposition of 
nutrient materials; and that it has therefore 
‘manifested itself in the body even during life. 
Again, when spontaneous gangrene occurs 
from obstruction to the circulation, decomposi- 
tion slowly supervenes in the part from which 
he ie supply of nutrient fluid is cut off; and 
_ coincident with its progress is the extinction of 
the vital properties, constituting molecular 
[ eath. (See vol. i. p. 791.) Corresponding 
changes may result in the whole body when 
the nutritive functions are interrupted, not by 
obs action to the motion of the circulating 
fi id, but by depravation of its character ; and 
we then perceive the vital properties of each 
t ssue impaired in a degree correspondent to 
the dependence of the integrity of its structure 
upon the constant renewal of its elements. 
_ The presumed impossibility of forming, by 
chemical combination of their elements, 
iy of the class of organic compounds or prox- 
principles, is regarded by many physiolo- 
gists as in itself a sufficient ground for the as- 
mption that the affinities which act in the 
living body are different from those which we 
‘recognize in the inorganic world. The fact, 
however, which we have already noticed re- 
arding the artificial production of urea is one 
ich powerfully opposes such an assump- 
tion.* This is slurred over by Miller, with 
ie remark that it can scarcely be considered 
as organic matter, being rather an excretion 
han a component of the body—a distinction 
which does not remove it from the pale of the 
tion of the supposed laws of vital affi- 
ity. Seeing the vast progress which organic 
chemistry has made during the last few years, 
d the rapid increase of our knowledge re- 
arding not merely the composition but the 
‘mutual relations of the class of bodies under 
consideration, we cannot but think it premature 
to assert that other compounds may not be pro- 
‘duced in a similar manner. Be it observed, 
however, that the doctrine for which we are 
ow arguing only concerns the production of 
compounds which are destined either to 
thrown off from the system, or to undergo 
Subsequent organisation ; and cannot apply to 
those in which the process of organisation, and 
@ consequent development of vital properties 
ve already commenced. This distinction is 
a yery important one, and may, we think, 
by being kept steadily in view, save much un- 
Successful because mis-directed labour. If, for 
aay our view be correct, it may be pos- 
ible for the chemist to produce the gum or 
Sugar which he finds in the ascending sap of 
. a: but he can never hope to imitate the 
atex or elaborated sap, which already shows 
) traces of organisation and of the possession of 
vital properties. In like manner the formation 
} of albumen may be a worthy object of his 
) endeavours, whilst these would be totally fruit- 
less if directed to the production of fibrin, 
- 
| 
_* We do not quote any others reported to possess 
| the same character, such as the production of fatty 
| matter by Berard and Hatchett, because they still 
| require confirmation. 
hers tlt, tN 
*™ 
an 
E LIFE. 
153 
which differs from it but little if at all in che- 
mical constitution, but which is endowed in 
its fluid state with properties that nothing but 
the influence of a living system can generate. 
But quitting these speculations, we shall in- 
quire what positive evidence may be produced 
of the operation of chemical affinities in the 
changes of composition that form so important 
a part of vital action. Many facts might be 
collected which favour such a belief; but the 
following must here suffice. In the progress 
of vegetation we have frequent oceasion to 
observe the conversion of gum and of fecula, 
which consists of gum enclosed in vesicles, 
into sugar. This takes place in germination, 
in the budding of the potato and other fleshy 
stems, in flowering, in the ripening of fruit, as 
well as in many other instances ; in alt these in 
which fecula is the subject of the change, it 
would seem that this product, having been 
stored away out of the current of the circu- 
lation against the time of need, is to be again 
brought into use, and to supply the pabulum 
of young or rapidly-growing parts by conver- 
sion into sugar. These changes are effected in 
various modes. Where gum is the subject of 
the conversion, we commonly find an acid 
employed to produce it, as in the ripening of 
fruits, where lignin as well as gum seems to 
undergo this change. The chemist can pro- 
duce the same effect by digesting gum or lignin 
with an acid at a certain temperature. Again, 
where the conversion of fecula into sugar takes 
place as one of the ordinary processes of the 
vegetable economy, it is effected by the pro- 
duction of a secretion termed diastase, which 
occasions both the rupture of the starch-vesicles 
and the change of their contained gum into 
sugar. This diastase, which is abundantly 
stored up in the neighbourhood of the eyes or 
buds of the potato, may be separately obtained 
by the chemist; and it acts as effectually in his 
laboratory as in that of the vegetable organism. 
Further, he can imitate its effects by other che- 
mical agents; for, by the joint operation of 
heat and acid, he can produce the same trans- 
formation. 
These are among the remarkable instances 
of the catalytic action recently described by 
Berzelius,* which is common to organie and 
inorganic operations, and which is not yet 
found to ke comprehensible within the known 
laws of chemical affinity. The peculiarity of 
the action consists in the production by one 
body, A, of a change in the composition of 
another, B C, without itself undergoing any 
alteration. Thus, the peroxide of hydrogen, 
which is readily decomposed by any substance 
having an affinity for oxygen, is also decom- 
posed by some which themselves undergo no 
change, such as the metals and the fibrin of the 
blood ; these produce in it a state analogous to 
fermentation, oxygen escaping and water being 
left. Again, not only decompositions but new 
combinations may be effected in this manner. 
Thus, most metals at high temperatures, and 
platinum in a state of minute division at low 
temperatures, as well as various porous sub- 
* Edinb. Phil. Journal, vol, xxi. 
