140 Organic Physics. [Fe 
motion contact (or its mental resultant) is evidently necessary. 
Solid substance, or vibratory energy, touches the surface of 
cell, or the extremity of conducting lines leading to the p 
plasmic cell. An immediate, definite motion occurs in response 
We call this sensitiveness, but a mere word explains nothin l 
The contact of external matter does not provide the motor 
displayed, but it in some way sets it free. The force is yielt 
instigated by some outward pressure or irritation. 
motion which results is related in position and direction to 
source of irritation. If this affect the whole cell the motio 
be general andindefinite. If it affect a limited region of the : 
the motion will be local and definite. 
‘In ascribing organic motions to chemical change we are 1 
giving unknown powers tochemism. Inorganic chemical s i 
yields numerous illustrative instances. Mass motion is a 
mon result of chemical action. When a substance changi 
of its molecules frequently takes place, causing visible mo 
and an eventual change of form in the mass. So the moven 
of cell protoplasm may arise from molecular rearrange! 
caused by chemical change ; and the so-called contraction of ! 
cular fibers is really but a change of form possibly due to 
same cause, a 
In like manner the influence of external irritation over 
motion is far from being peculiar. It has many parallels 1 
inorganic world, of which a few may be here mentioned. 
ganic substances which combine but slowly, or not at all, 
mixed and undisturbed, often combine rapidly and even 
sively when exposed to external irritation. The cause 
sudden manifestation of chemical affinity is probably a vi 
which flings the molecules together, and thus aids their afii 
In the case of gunpowder, vigorous heat vibration causes 
and explosive combination of molecules. In other mixt 
vibration produced by a blow yield a like result. In lik 
bined in the dark, combines explosively if exposed to the vib 
| inlluence of sunlight. In all these cases mass motions: 
fined. Ayet mòre significant instance of sensitiveness t 
