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of Edinburgh , Session 1868 - 69 . 
tion of its substance, and by the consequent liberation of free 
transmissible force. 
4 th ground. The circumstance that the complex albuminous and 
oleaginous constitution of the cerebral substance yields readily to 
decomposition, and that it is the part whose chemical combinations 
are the first to break up after death, serves further to strengthen 
the views here set forth. 
The subject is, doubtless, one very difficult to in vestigate ; for 
even if, in the face of much contradictory evidence, we accept the 
opinion that the amount of Urea and other nitrogenous products 
discharged from the body affords a correct measure of the amount 
of animal work performed in a given time, yet from the circum- 
stance that both the brain and the muscles are chiefly nitrogenous 
in their constitution, it must ever remain difficult, if not impossible, 
by any mere examination of the nitrogenous excretions, to ascer- 
tain whether they are derived from the one organ or from the other, 
and therefore, the most ingenious and careful investigations will be 
required before we are entitled to dispossess the brain of what 
appears, on pretty strong grounds, to be one of its most important 
functions, viz., that of being the organ to the mind for the pro- 
duction of animal motion and power. 
On Perception. 
The second part of this paper has reference to the subject of 
sense-perception, or the impressions which the mind obtains by 
reason of its connection with the physical world. 
The writer directs attention to the fact that an examination of 
the laws of momentum, and of the transmission of the rapid con- 
cussions and vibrations which pass through the atmospheric and 
ethereal media, makes it apparent that free force or power is trans- 
mitted from molecule to molecule of these media, and that, philo- 
sophically speaking, it is this free force which thus enters the 
brain through the appropriate nerves, and which impresses the mind 
with the sensations of light and sound. 
The difficulty which has hitherto been felt by philosophers, not 
only of the various continental schools, but also of our Scottish 
school of philosophy, has ever been — how matter or physical sub- 
stance could act on mind ? This difficulty prompted Malebranche 
