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. CHAPTER XIV INORGANIC RESPONSE--FATIGUE, STAIRCASE, AND MODIFIED RESPONSE Fatigue in metals--Fatigue under continuous stimulation--Staircase effect--Reversed responses due to molecular modification in nerve and metal, and their transformation into normal after continuous stimulation--Increased response after continuous stimulation. [Illustration: FIG. 69.--FATIGUE IN MUSCLE (WALLER)] [Illustration: FIG. 70.--FATIGUE IN PLATINUM] #Fatigue.#--In some metals, as in muscle and in plant, we find instances of that progressive diminution of response which is known as fatigue (fig. 69). The accompanying record shows this in platinum (fig. 70). It has been said that tin is practically indefatigable. We must, however, remember that this is a question of degree only. Nothing is absolutely indefatigable. The exhibition of fatigue depends on various conditions. Even in tin, then, I obtained the characteristic fatigue-curve with a specimen which had been in continuous use for many days (fig. 71). While discussing the subject of fatigue in plants, I have adduced considerations which showed that the residual effect of strain was one of the main causes for the production of fatigue. This conclusion receives independent support from the records obtained with metals. [Illustration: FIG. 71.--FATIGUE SHOWN BY TIN WIRE WHICH HAD BEEN CONTINUOUSLY STIMULATED FOR SEVERAL DAYS]