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PROFESSOR TYNDALL. I29needle-dip from an already affected tube the resulting contagion was almostimmediate, while where dust was supplied in its dry form, two days mostlyelapsed before any indication of such inoculation made its appearance. Thesagacious explanation of these phenomena, given by Professor Tyndall, wasthat the dust supplied contained only germs in a desiccated state, whichnecessarily required a set time, or "period of latency," to expire beforethey displayed their vital properties while in those taken from the fluid;medium, these vital properties were already in their full force, permittingthe organisms to increase and multiply from the first moment of theircontact with the sterile liquid.One interesting experiment bearing upon the phenomenonlast describedrequires mention. A certain mineral solution, containing in proper proportionsall the substances which enter into the composition of Bacteria, wasfound after inoculation with the least speck of liquid containing livingBacteria, to be always swarming and turbid with such organisms within aspace of twenty-four hours while a small;pinch of laboratory dust added tothe same fluid, and containing the germs in their desiccated condition,remained in contact with the fluid with impunity for many weeks. Bacteriain their living and moist condition, and those in their desiccated state, werethus shown to possess highly differentiated developmental properties.Another fact of importance, elicited by Professor Tyndall, bears referenceto the want of uniformity in the distribution of bacterial and other germs inany given atmosphere. This was demonstrated through the preparation oflarge trays, contrived to hold as many as from sixty to one hundred tubesof infusions side by side, and on the same level. All of these exposed todust-laden air were infallibly, after a greater or less duration of time, teemingwith living organisms, but the order of their affection or inoculation wasfound to differ considerably, intervals of several days not unfrequentlyelapsing between the inoculation of closely contiguous tubes. A considerabledifference was likewise found to obtain, under such conditions, in thecharacter of the developed matter, Bacteria of different species, fungoidgrowths, and other organisms, variously and irregularly preponderating.Professor Tyndall happily explains these phenomena by comparing theaerial distribution of microscopic germs to the cloud-patchesmottled sky ;visible in aall parts of the landscape, as represented by the tray of tubes,being overshadowed in turn by these patches, but in no definite or regularsequence. It has been pointed out by Professor Huxley, that a closelycorresponding simile was originally employed by Ehrenberg, who as anexponent of the atmospheric distribution of Infusoria, either as eggs or intheir encysted state, likened the non-uniformity of their occurrence undersuch conditions to irregularly alternating days of sunshine and heavydownpour. As shown already, however, at page 120, the atmospheric germtheory originated with John Harris, more than a century prior even to thetime of Ehrenberg.That the atmosphere inits purest state may be entirely free from organicK