§ 3. Decay of Memory with Lapse of Time. — Though particular memories last for various periods in different cases and with different persons, yet it is the general law that they tend to die away in course of time if they are not refreshed. Professor Ebbinghaus has made experiments with the view of determining the quantitative relation between lapse of time and decay of the power of recall. For this purpose he learnt by heart lists of unmeaning syllables of three letters each; each list contained from twelve to thirtysix syllables. After learning a list so as to be able to repeat it, an interval of time was allowed to intervene before again attempting to recall the syllables. Memory had in the interim become more or less partial and fragmentary. The point of the experiment was to determine the amount of time required for relearning the list as compared with the time originally required. This yields a measure of the degree of decay of the mental dispositions, and shows the relation between decay and lapse of time. After an interval of 20 minutes, about 40 per cent of the original time was required for relearning, after 64 minutes, about 56 per cent, after 526 minutes, about 65 per cent, after two days, about 72 per cent, and so on. From this we see that though the amount of decay increases with the lapse of time, yet relatively it is smaller the longer the interval.