PROFESSOR TYNDALL. I 29 



needle-dip from an already affected tube the resulting contagion was almost 

 immediate, while where dust was supplied in its dry form, two days mostly 

 elapsed before any indication of such inoculation made its appearance. The 

 sagacious explanation of these phenomena, given by Professor Tyndall, was 

 that the dust supplied contained only germs in a desiccated state, which 

 necessarily required a set time, or " period of latency," to expire before 

 they displayed their vital properties ; while in those taken from the fluid 

 medium, these vital properties were already in their full force, permitting 

 the organisms to increase and multiply from the first moment of their 

 contact with the sterile liquid. 



One interesting experiment bearing upon the phenomenon last described 

 requires mention. A certain mineral solution, containing in proper propor- 

 tions all the substances which enter into the composition of Bacteria, was 

 found after inoculation with the least speck of liquid containing living 

 Bacteria, to be always swarming and turbid with such organisms within a 

 space of twenty-four hours ; while a small pinch of laboratory dust added to 

 the same fluid, and containing the germs in their desiccated condition, 

 remained in contact with the fluid with impunity for many weeks. Bacteria 

 in their living and moist condition, and those in their desiccated state, were 

 thus shown to possess highly differentiated developmental properties. 

 Another fact of importance, elicited by Professor Tyndall, bears reference 

 to the want of uniformity in the distribution of bacterial and other germs in 

 any given atmosphere. This was demonstrated through the preparation of 

 large trays, contrived to hold as many as from sixty to one hundred tubes 

 of infusions side by side, and on the same level. All of these exposed to 

 dust-laden air were infallibly, after a greater or less duration of time, teem- 

 ing with living organisms, but the order of their affection or inoculation was 

 found to differ considerably, intervals of several days not unfrequently 

 elapsing between the inoculation of closely contiguous tubes. A consider- 

 able difference was likewise found to obtain, under such conditions, in the 

 character of the developed matter. Bacteria of different species, fungoid 

 growths, and other organisms, variously and irregularly preponderating. 

 Professor Tyndall happily explains these phenomena by comparing the 

 aerial distribution of microscopic germs to the cloud-patches visible in a 

 mottled sky ; all parts of the landscape, as represented by the tray of tubes, 

 being overshadowed in turn by these patches, but in no definite or regular 

 sequence. It has been pointed out by Professor Huxley, that a closely 

 corresponding simile was originally employed by Ehrenberg, who as an 

 exponent of the atmospheric distribution of Infusoria, either as eggs or in 

 their encysted state, likened the non-uniformity of their occurrence under 

 such conditions to irregularly alternating days of sunshine and heavy 

 downpour. As shown already, however, at page 120, the atmospheric germ 

 theory originated with John Harris, more than a century prior even to the 

 time of Ehrenberg. 



That the atmosphere in its purest state may be entirely free from organic 



K 



