Is the continuance of the movements of the organisms which had been boiled attributable to their extreme lightness, and to the slight difference between their specific gravity and that of the fluid in which they are immersed? I soon became convinced that this was one, if not the chief reason, when I found that _Bacteria_ which had been submitted to very much higher temperatures, behaved in precisely the same manner as those which had been merely boiled, and also that other particles which--though obviously dead--had a similar specific lightness, also continued to exhibit their Brownian movements for days and weeks. This was the case more especially with the minute fat particles in a mounted specimen of boiled milk,[29] and also with very minute particles which were gradually precipitated[30] from a hay infusion that had been heated to 302° F. for four hours. Trials with many different substances, indeed, after a time convinced me that the most rapid cessation of Brownian movements in stationary films,[31] occurred where the particles were heavy or large; and that the duration of the movement was more and more prolonged, as the particles experimented with, were lighter or more minute. So that, when we have to do with _Bacteria_, the minute oil globules of milk, or with other similarly light particles, the movements continue for an indefinite time, and are, in part, mere exponents of the molecular unrest of the fluid. They are always capable of being increased or renewed by the incidence of heat or other disturbing agencies. [29] If an unboiled specimen of milk be mounted, a multiplication of living particles takes place here and there amongst the fat globules, just as the multiplication of _Bacteria_ occurs in a vegetable infusion; but in the boiled specimen no trace of such multiplication can ever be detected. [30] Those particles which come to rest, in such cases, are always in contact with one or other of the contiguous surfaces of glass. [31] The specific gravity of the fluid being constant. Where this is dense or viscid, as with glycerine, Brownian movements do not occur at all. In respect of the movements which they may exhibit, therefore, really living, though languid, _Bacteria_, cannot always be discriminated from dead _Bacteria_. Both may only display mere Brownian movements. It becomes obvious, then, that in doubtful cases we ought not to rely very strongly upon the character of their movements, as evidence of the death of _Bacteria_--although these may frequently be of so extensive a nature as to render it not at all doubtful whether the _Bacteria_ which display them are living. In the experiments which I am about to relate, we shall be able to pronounce that the _Bacteria_ are living or dead, by reference to the continuance or cessation of their most essentially vital characteristic. If _Bacteria_ fail to multiply in a suitable fluid, and under suitable conditions, we have the best proof that can be obtained of their death.