Page:Collected Physical Papers.djvu/405

Rh Certain chemical substances produce a great enhancement in the amplitude of response; these act as stimulants in inducing an increase of excitability (p. 274). Others produce a depression. The variation of excitability induced by various chemical substances, or different doses of the same substance can be detected by the Electric Comparator (p. 282). Slight differences of physico-chemical change in the same piece of metal are detected and recorded by the Electra-molecular Explorer (p. 287).

The effect of an identical chemical agent is modified by the dose of application, a minute dose often producing a reaction opposite to that of a large dose (p. 301). Poisons like oxalic acid cause a molecular arrest and 'kill' the response of metals (p. 303).

Among the manifestations in the life of the plant may be mentioned its irritability for response to external stimulation, its growth and its power of storing energy supplied by the environment.

The power of response in plants has generally been regarded as confined to the sensitives like Mimosa pudica. Experimental investigations are described which prove that all plants and their different organs are fully sensitive, and that the characteristic electric response by induced galvanometric negativity given by them is in every way similar to the electric response of animal tissues (p. 309).

Successive equal stimulations give rise to uniform responses. Shortening of the intervening period of