It would be a most important step if we could ascertain some means by which these primary movements of living _Bacteria_ might be distinguished from the secondary, or communicated, movements of not-living particles. In many cases, organisms that are truly living may only exhibit very languid movements, which, as movements, are quite indistinguishable from those that the same _Bacteria_ may display when they are really dead. Because the movements, therefore, are of this doubtful character, persons are apt, unfairly, to argue that the _Bacteria_ which present them, are no more living than are the minute particles of carbon obtained from the flame of a lamp, which may exhibit similar movements. This, however, is a point of view which becomes obviously misleading if too much stress is laid upon it; and it is more especially so in this case, when those _Bacteria_ which display the most characteristic sign of vitality--viz., “spontaneous” division or reproduction--do, at the time, almost always exhibit only the same languid movements. Mobility is, in fact, not an essential characteristic of living _Bacteria_, whilst _the occurrence of the act of reproduction is the most indubitable sign of their life_. It should be remembered, therefore, that any _Bacteria_ which are almost motionless, or which exhibit mere Brownian movements, _may_ be living, whilst those which spontaneously divide and reproduce, are certainly alive--whatever may be the kind of movement they present. In any particular case, however, can we decide whether _Bacteria_, that have been submitted to a given temperature, and which exhibit movements resembling those known as Brownian, are really dead or living? If the movements are primary, or dependent upon the inherent molecular activity of the organism itself, they ought, it might be argued, to continue when the molecules of the fluid are at rest; if, on the other hand, they are mere secondary or communicated movements, impressed upon the organisms as they would be upon any other similarly minute particles, by the molecular oscillations of the fluid in which they are contained, then the movements ought to grow less, and gradually cease, as the fluid approaches a state of molecular rest--if this be attainable. Following out this idea, some months ago, I first tested the correctness of the assumption by experimenting with fluids containing various kinds of not-living particles; such as carbon-particles from the flame of a lamp, or freshly precipitated baric sulphate. However perfect may have been the Brownian movements when portions of these fluids were first examined beneath a covering-glass, they always gradually diminished, after the specimen had been mounted by surrounding the covering-glass with some cement or varnish. Thus prepared, no evaporation could take place from the thin film of fluid, and after one, three, four, or more hours--the slide remaining undisturbed--most of the particles had subsided, and