INORGANIC RESPONSE--MOLECULAR MOBILITY: ITS INFLUENCE ON RESPONSE
Effects of molecular inertia--Prolongation of period of recovery by
overstrain--Molecular model--Reduction of molecular sluggishness
attended by quickened recovery and heightened response--Effect of
temperature--Modification of latent period and period of recovery by
the action of chemical reagents--Diphasic variation.
We have seen that the stimulation of matter causes an electric
variation, and that the acted substance gradually recovers from the
effect of stimulus. We shall next study how the form of response-curves
is modified by various agencies.
In order to study these effects we must use, in practice, a highly
sensitive galvanometer as the recorder of E.M. variations. This
necessitates the use of an instrument with a comparatively long period
of swing of needle, or of suspended coil (as in a D'Arsonval). Owing to
inertia of the recording galvanometer, however, there is a lag produced
in the records of E.M. changes. But this can be distinguished from the
effect of the molecular inertia of the substance itself by comparing two
successive records taken with the same instrument, in one of which the
latter effect is relatively absent, and in the other present. We wish,
for example, to find out whether the E.M. effect of mechanical stimulus
is instantaneous, or, again, whether the effect disappears immediately.
We first take a galvanometer record of the sudden introduction and
cessation of an E.M.F. on the circuit containing the vibration-cell
(fig. 60, _a_). We then take a record of the E.M. effect produced by a
stimulus caused by a single torsional vibration. In order to make the
conditions of the two experiments as similar as possible, the disturbing
E.M.F., from a potentiometer, is previously adjusted to give a
deflection nearly equal to that caused by stimulus. The torsional
vibration was accomplished in a quarter of a second, and the contact
with the potentiometer circuit was also made for the same length of
time.
[Illustration: FIG. 60
(_a_) Arrangement for applying a short-lived E.M.F.
(_b_) Difference in the periods of recovery: (1) from instantaneous
E.M.F.; and (2) that caused by mechanical stimulus.]
The record was then taken as follows. The recording drum had a fast
speed of six inches in a minute, one of the small subdivisions
representing a second. The battery contact in the main potentiometer
circuit was made for a quarter of a second as just mentioned and a
record taken of the effect of a short-lived E.M.F. on the circuit
containing the cell. (2) A record was next taken of the E.M. variation
produced in the cell by a single stimulus. It will be seen on comparison
of the two records that the maximum effect took place relatively later
in the case of mechanical stimulus, and that whereas the galvanometer
recovery in the former case took place in 11 seconds, the recovery in
the latter was not complete till after 60 seconds (fig. 60, _b_). This
shows that it takes some time for the effect of stimulus to attain its
maximum, and that the effect does not disappear till after the lapse of
a certain interval. The time of recovery from strain depends on the
intensity of stimulus. It takes a longer time to recover from a stronger
stimulus. But, other things being equal, successive recovery periods
from successive stimulations of equal intensity are, generally speaking,
the same.
We may now study the influence of any change in external conditions by
observing the modifications it produces in the normal curve.
[Illustration: FIG. 61.--PROLONGATION OF PERIOD OF RECOVERY AFTER
OVERSTRAIN
Recovery is complete in 60" when the stimulus is due to 20° vibration.
But with stronger stimulus of 40° vibration, the period of recovery
is prolonged to 90".]
#Prolongation of period of recovery by overstrain.#--The pair of records
given in fig. 61 shows how recovery is delayed, as the effect of
overstrain. Curve (_a_) is for a single stimulus due to a vibration of
amplitude 20°, and curve (_b_) for a stimulus of 40° amplitude of
vibration. It will be noticed how relatively prolonged is the recovery
in the latter case.
[Illustration: FIG. 62.--MODEL SHOWING THE EFFECT OF FRICTION]
#Molecular Model.#--We have seen that the electric response is an outward
expression of the molecular disturbance produced by the action of the
stimulus. The rising part of the response-curve thus exhibits the effect
of molecular upset, and the falling part, or recovery, the restoration
to equilibrium. The mechanical model (fig. 62) will help us to visualise
many complex response phenomena. The molecular model consists of a
torsional pendulum--a wire with a dependent sphere. By the stimulus of a
blow there is produced a torsional vibration--a response followed by
recovery. The writing lever attached to the pendulum records the
response-curves. The form of these curves, stimulus remaining constant,
will be modified by friction; the less the friction, the greater is the
mobility. The friction may be varied by more or less raising a vessel of
sand touching the pendulum. By varying the friction the following curves
were obtained.
(_a_) When there is little friction we get an after-oscillation, to
which we have the corresponding phenomenon in the retinal
after-oscillation (compare fig. 105).
(_b_ and _c_) If the friction is increased, there is a damping of
oscillation. In (_c_) we get recovery-curves similar to those found in
nerve, muscle, plant, and metal.
(_d_) If the friction is still further increased the maximum is reached
much later, as will be seen in the increasing slant of the rising part
of the curve; the height of response is diminished and the period of
recovery very much prolonged by partial molecular arrest. The curve
(_d_) is very similar to the 'molecular arrest' curve obtained by small
dose of chemical reagents which act as 'poison' on living tissue or on
metals (compare fig. 93, _a_).
(_e_) When the molecular mobility is further decreased there is no
recovery (compare fig. 93, _b_).
Still further increase of friction completely arrests the molecular
pendulum, and there is no response.
From what has been said, it will be seen that if in any way the friction
is diminished or mobility increased the response will be enhanced. This
is well exemplified in the heightened response after annealing (fig. 58)
and after preliminary vibration (figs. 81, 82).
Possibly connected with this may be the increased responses exhibited by
the action of stimulants (figs. 89, 90).
#Reduction of molecular sluggishness attended (1) by quickened
recovery.#--Sometimes, after a cell has been resting for too long a
period, especially on cold days, the wire gets into a sluggish
condition, and the period of recovery is thereby prolonged. But
successive vibrations gradually remove this inertness, and recovery is
then hastened. This is shown in the accompanying curves, fig. 63, where
(_a_) exhibits only very partial recovery even after the expiration of
60 seconds, whereas when a few vibrations had been given recovery was
entirely completed in 47 seconds (_b_). There was here little change in
the height of response.
[Illustration: FIG. 63
(_a_) Slow recovery of a wire in a sluggish condition.
(_b_) Quickened recovery in the same wire after a few vibrations.]
#Or (2) by heightened response.#--The removal of sluggishness by
vibration, resulting in increased molecular mobility, is in other
instances attended by increase in the height of response, as will be
seen from the two sets of records which follow (fig. 64). Cold, due to
prevailing frosty weather, had made the wires in the cell somewhat
lethargic. The records in (_a_) were the first taken on the day of the
experiment. The amplitudes of vibration were 45°, 90°, and 135°. In
(_b_) are given the records of the next series, which are in every case
greater than those of (_a_). This shows that previous vibration, by
conferring increased mobility, had heightened the response. In this
case, removal of molecular sluggishness is attended by greater intensity
of response, without much change in the period of recovery. In
connection with this it must be remembered that greater strain
consequent on heightened response has a general tendency to a
prolongation of the period of recovery.
[Illustration: FIG. 64
(_a_) Three sets of responses for 45°, 90°, and 135° vibration in a
sluggish wire.
(_b_) The next three sets of responses in the same wire; increased
mobility conferred by previous vibration has heightened the
response.]
It is thus seen that when the wire is in a sluggish condition,
successive vibrations confer increased molecular mobility, which finds
expression in quickened recovery or heightened response.
#Effect of temperature.#--Similar considerations lead us to expect that a
moderate rise of temperature will be conducive to increase of response.
This is exhibited in the next series of records. The wire at the low
temperature of 5° C. happened to be in a sluggish condition, and the
responses to vibrations of 45° to 90° in amplitude were feeble. Tepid
water at 30° C. was now substituted for the cold water in the cell, and
the responses underwent a remarkable enhancement. But the excessive
molecular disturbance caused by the high temperature of 90° C. produced
a great diminution of response (fig. 65).
[Illustration: FIG. 65.--RESPONSES OF A WIRE TO AMPLITUDES OF VIBRATION
45° AND 90°
(_a_) Responses when the wire was in a sluggish condition at temperature
of 5° C.
(_b_) Enhanced response at 30° C.
(_c_) Diminution of response at 90° C.]
#Diphasic variation.#--It has already been said that if two points A and B
are in the same physico-chemical condition, then a given stimulus will
give rise to similar excitatory electric effects at the two points. If
the galvanometer deflection is 'up' when A alone is excited, the
excitation of B will give rise to a downward deflection. When the two
points are simultaneously excited the electric variation at the two
points will _continuously_ balance each other. Under such conditions
there will be no resultant deflection. But if the intensity of
stimulation of one point is relatively stronger, then the balance will
be disturbed, and a resultant deflection produced whose sign and
magnitude can be found independently by the algebraical summation of the
individual effects of A and B.
It has also been shown that a balancing point for the block, which is
approximately near the middle of the wire, may be found so that the
vibrations of A and B through the same amplitude produce equal and
opposite deflection. Simultaneous vibration of both will give no
resultant current; when the block is abolished and the wire is vibrated
as a whole, there will still be no resultant, inasmuch as similar
excitations are produced at A and B.
After obtaining the balance, if we apply an exciting reagent like
Na_2CO_3 at one point, and a depressing reagent like KBr at the
other, the responses will now become unequal, the more excitable point
giving a stronger deflection. We can, however, make the two deflections
equal by increasing the amplitude of vibration of the less sensitive
point. The two deflections may thus be rendered equal and opposite, but
the time relations--the latent period, the time rate for attaining the
maximum excitation and recovery from that effect--will no longer be the
same in the two cases. There would therefore be no continuous balance,
and we obtain instead a very interesting diphasic record. I give below
an exact reproduction of the response-curves of A and B recorded on a
fast-moving drum. It will be remembered that one point was touched with
Na_2CO_3 and the other with KBr. By suitably increasing the
amplitude of vibration of the less sensitive, the two deflections were
rendered approximately equal. The records of A and B were at first taken
separately (fig. 66, _a_). It will be noticed that the maximum
deflection of A was attained relatively much earlier than that of B.
The resultant curve R' was obtained by summation.
[Illustration: FIG. 66.--DIPHASIC VARIATION
(_a_) Records of A and B obtained separately. R' is the resultant by
algebraical summation. (_b_) Diphasic record obtained by
simultaneous stimulation of A and B.]
After taking the records of A and B separately, a record of resultant
effect R due to simultaneous vibration of A and B was next taken. It
gave the curious two-phased response--positive effect followed by
negative after-vibration, practically similar to the resultant curve R'
(fig. 66, _b_).
The positive portion of the curve is due to A effect and the negative to
B. If by any means, say by either increasing the amplitude of vibration
of A or increasing its sensitiveness, the response of A is very greatly
enhanced, then the positive effect would be predominant and the negative
effect would become inconspicuous. When the two constituent responses
are of the same order of magnitude, we shall have a positive response
followed by a negative after-vibration; the first twitch will belong to
the one which responds earlier. If the response of A is very much
reduced, then the positive effect will be reduced to a mere twitch and
the negative effect will become predominant.
I give a series of records, fig. 67, in which these three principal
types are well exhibited, the two contacts having been rendered
unequally excitable by solutions of the two reagents KBr and
Na_2CO_3. A and B were vibrated simultaneously and records taken.
(_a_) First, the relative response of B (downward) is increased by
increasing its amplitude of vibration. The amplitude of vibration of A
was throughout maintained constant. The negative or downward response is
now very conspicuous, there being only a mere preliminary indication of
the positive effect. (_b_) The amplitude of vibration of B is now
slightly reduced, and we obtain the diphasic effect. (_c_) The intensity
of vibration of B is diminished still further, and the negative effect
is seen reduced to a slight downward after-vibration, the positive
up-curve being now very prominent (fig. 67).
[Illustration: FIG. 67.--NEGATIVE, DIPHASIC, AND POSITIVE RESULTANT
RESPONSE]
#Continuous transformation from negative to positive.#--I have shown
the three phases of transformation, the intensity of one of the
constituent responses being varied by altering the intensity of
disturbance.
In the following record (fig. 68) I succeeded in obtaining a continuous
transformation from positive to negative phase by a continuous change in
the relative sensitiveness of the two contacts.
I found that traces of after-effect due to the application of
Na_2CO_3 remain for a time. If the reagent is previously applied to
an area and the traces of the carbonate then washed off, the increased
sensitiveness conferred disappears gradually. Again, if we apply
Na_2CO_3 solution to a fresh point, the sensitiveness gradually
increases. There is another further interesting point to be noticed: the
beginning of response is earlier when the application of Na_2CO_3 is
fresh.
[Illustration: FIG. 68.--CONTINUOUS TRANSFORMATION FROM NEGATIVE TO
POSITIVE THROUGH INTERMEDIATE DIPHASIC RESPONSE
Thick dots represent the times of application of successive stimuli.]
We have thus a wire held at one end, and successive uniform vibrations
at intervals of one minute imparted to the wire as a whole, by means of
a vibration head on the other end.
Owing to the after-effect of previous application of Na_2CO_3 the
sensitiveness of B is at the beginning great, hence the three resultant
responses at the beginning are negative or downward.
Dilute solution of Na_2CO_3 is next applied to A. The response of A
(up) begins earlier and continues to grow stronger and stronger. Hence,
after this application, the response shows a preliminary positive twitch
of A followed by negative deflection of B. The positive grows
continuously. At the fifth response the two phases, positive and
negative, become equal, after that the positive becomes very prominent,
the negative being reduced as a feeble after-vibration.
It need only be added here that the diphasic variations as exhibited by
metals are in every way counterparts of similar phenomena observed in
animal tissues.