The instances, according to my personal experience, are very rare, and even those are not very satisfactory, in which some event recalls a memory that had lain absolutely dormant for many years.  In this very series of experiments a recollection which I thought had entirely lapsed appeared under no less than three different aspects on different occasions.  It was this:  when I was a boy, my father, who was anxious that I should learn something of physical science, which was then never taught at school, arranged with the owner of a large chemist’s shop to let me dabble at chemistry for a few days in his laboratory.  I had not thought of this fact, so far as I was aware, for many years; but in scrutinising the fleeting associations called up by the various words, I traced two mental visual images (an alembic and a particular arrangement of tables and light), and one mental sense of smell (chlorine gas) to that very laboratory.  I recognised that these images appeared familiar to me, but I had not thought of their origin.  No doubt if some strange conjunction of circumstances had suddenly recalled those three associations at the same time, with perhaps two or three other collateral matters which may be still living in my memory, but which I no not as yet identify, a mental perception of startling vividness would be the result, and I should have falsely imagined that it had supernaturally, as it were, started into life from an entire oblivion extending over many years.  Probably many persons would have registered such a case as evidence that things once perceived can never wholly vanish from the recollection, but that in the hour of death, or under some excitement, every event of a past life may reappear.  To this view I entirely dissent.  Forgetfulness appears absolute in the vast majority of cases, and our supposed recollections of a past life are, I believe, no more than that of a large number of episodes in it, to be reckoned perhaps in hundreds of thousands, but certainly not in tens of hundreds of thousands, that have escaped oblivion.  Every one of the fleeting, half-conscious thoughts that were the subject of my experiments, admitted of being vivified by keen attention, or by some appropriate association, but I strongly suspect that ideas which have long since ceased to fleet through the brain, owing to the absence of current associations to call them up, disappear wholly.  A comparison of old memories with a newly-met friend of one’s boyhood, about the events we then witnessed together, show how much we had each of us forgotten.  Our recollections do not tally.  Actors and incidents that seem to have been of primary importance in those events to the one have been utterly forgotten by the other.  The recollection of our earlier years are, in truth, very scanty, as any one will find who tries to enumerate them.