Page:Collected Physical Papers.djvu/322

302 Opposite Effects of strong and feeble dose.—The most curious effect is that exhibited by the same reagent when the strength of solution is varied. This is clearly seen in record (fig. 82), in which (a) gives the normal response in water; (b)) shows the enhancement of response by a highly dilute solution of KHO, 3 parts in a thousand. The response was completely abolished by a stronger solution, 3 parts in a hundred (fig. 82 c).

Effect of "poisons."—Certain agents like oxalic acid cause a total abolition of response, like the action of poisons on living tissues.

The facts described above appear to show that the enhancement or depression of response may, at least to a considerable extent, be due to the increase or diminution of molecular mobility. With a given stimulus, the height of response and the form of the response curve is determined by the element of molecular friction. In connection with this, it is instructive to observe the records of vibrations of a torsional pendulum, the friction being gradually increased by immersing the pendulum in a viscous fluid. The various types of response-curves in metals are found to be very similar to the above.

Of these I give an interesting example. With moderate friction the successive curves obtained with the pendulum are like those given in the left of fig. 83 (a). With increased friction the height of the curve is diminished, the maximum is reached later, and the recovery becomes much prolonged like the curve to the right. With still greater friction there is an arrest of recovery. It would appear as if the reagents which abolish response in metals produce a similar molecular arrest.