We have already seen that metals respond to stimulus by E.M. variation, just as do animal and vegetable tissues. We have yet to see whether the similarity extends to this point only, or goes still further, whether the response-curves of living and in organic are alike, and whether the inorganic response-curve is modified, as living response was found to be, by the influence of external agencies. If so, are the modifications similar? What are the effects of superposition of stimuli? Is there fatigue? If there be, in what way does it affect the curves? And lastly, is the response of metals exalted or depressed by the action of chemical reagents? #Conditions of obtaining quantitative measurements.#--In order to carry out these investigations, it is necessary to remove all sources of uncertainty, and obtain quantitative measurements. Many difficulties at first presented themselves in the course of this attempt, but they were completely removed by the adoption of the following experimental modification. In the simple arrangement for qualitative demonstration of response in metals previously described, successive experiments will not give results which are strictly comparable (1) unless the resistance of the circuit be maintained constant. This would necessitate the adoption of some plan for keeping the electrolytic contacts at A and B absolutely invariable. There should then be no chance of any shifting or variation of contact. (2) There must also be some means of applying successive stimuli of equal intensity. (3) And for certain further experiments it will be necessary to have some way of gradually increasing or decreasing the stimuli in a definite manner. #Modification of the block method.#--By consideration of the following experimental modifications of the block method (fig. 55), it will be found easy to construct a perfected form of apparatus, in which all these conditions are fully met. The essentials to be kept in mind were the introduction of a complete block midway in the wire, so that the disturbance of one half should be prevented from reaching the other, and the making of a perfect electrolytic contact for the electrodes leading to the galvanometer. Starting from the simple arrangement previously described where a straight wire is clamped in the middle (fig. 55, _a_), we next arrive at (_b_). Here the wire A B is placed in a U tube and clamped in the middle by a tightly fitting cork. Melted paraffin wax is poured to a certain depth in the bend of the tube. The two limbs of the tube are now filled with water, till the ends A and B are completely immersed. Connection is made with the non-polarisable electrodes by the side tubes. Vibration may be imparted to either A or B by means of ebonite clip holders seen at the upper ends A B of the wire.