Thus protected, evaporation takes place very slowly, and with the live-box resting on one of Stricker’s hot-water plates, at a temperature of 85° to 90° F., and the latter upon the stage of the microscope, one can easily select a portion of the field in which either no particles or only a countable number exist. If, therefore, around and between any mere granules which may pre-exist, or in a clear space, one gradually sees in the course of two or perhaps three hours, a multitude of almost motionless specks (at first about 1/100000″ in diameter) in positions where no such specks previously existed; and if these specks may be seen gradually to increase in size and develop into _Bacteria_ and _Torulæ_, then, at all events, we are able to say that these organisms can be developed without pre-existing _visible_ germs, and we have just the same amount of actual evidence for believing that they have been formed _de novo_, as we should have for believing that crystals had been formed _de novo_, if we had seen them appearing under our eyes in the same manner. Whether they really arise after the fashion of crystals, without the aid of pre-existing though invisible germs, is a matter which can only be settled inferentially, by a subsequent resort to strict methods of experimentation. Seeing however, that we are able, with the aid of the microscope alone, to demonstrate that _Bacteria_ and _Torulæ_ can develop in situations where no visible germs had previously existed, it is useless, as I have said before--so far as the question of their mode of origin is concerned--to search the atmosphere to ascertain what visible germs it may contain. If some _Bacteria_ and _Torulæ_ arise from germs at all, it must be from germs which are invisible to us. The finding of visible germs in the atmosphere can, therefore, only have an indirect bearing upon the solution of the problem. Since it _can_ be shown that some visible spores and ova exist in the atmosphere, this affords a certain amount of warrant for the supposition that invisible, living, reproductive particles may also exist--more especially if the existence of an amount of organic matter, which is ordinarily invisible, can be revealed in the air, by the agency of the electric beam, or by any other means.