xxxi 



mutton, hare, rabbit, kidney, liver, fowl, pheasant, grouse, haddock,, 

 sole, salmon, cod, turbot, mullet, herring, whiting, eel, and oysters. 

 The result was that infusions of these substances, exposed to temper- 

 atures varying from 27 C. to 32 C. in these germless atmospheres, 

 in no single instance underwent putrefaction or developed the 

 slightest amount of bacterial life. On the other hand, infusions of the 

 same substances, exposed to the common air of the Royal Institution 

 laboratory, all fell into putrefaction in the course of from 2 to 4 days ; 

 no matter where the infusions were placed, they infallibly became 

 offensive in the end. The number of tubes containing infusions was 

 multiplied until it reached 600, but not one of them escaped infection. 

 To detect the floating germs in the air, Tyndall employed a power- 

 ful beam of light from which the eye of the observer was carefully 

 screened. " When the track of a parallel beam in dusty air," says 

 Tyndall, " is looked at horizontally through a Nicol's prism, in a 

 direction perpendicular to the beam, the longer diagonal of the 

 prism being vertical, a considerable portion of the light from the 

 finest portions of the suspended matter is extinguished. The coarser 

 motes, on the other hand, flash out with greater force, because of the 

 increased darkness of the space around them. It is among the finest 

 ultra-microscopic particles that the matter potential as regards the 

 development of bacterial life is to be sought." He was thus em- 

 ploying for the detection of suspended matter in air an instrument 

 far more delicate than the microscope, and he reasons upon the 

 results of his experiments as follows : " But though they are beyond 

 the reach of the microscope, the existence of these particles, foreign 

 to the atmosphere, but floating in it, is as certain as if they could be 

 felt between the fingers, or seen by the naked eye. Supposing them 

 to augment in magnitude until they come, not only within the range 

 of the microscope, but within range of the unaided senses ; let it be 

 assumed that our knowledge of them under these circumstances 

 remains as defective as it is now that we do not know whether they 

 are germs, particles of dead organic dust, or particles of mineral 

 matter. Suppose a vessel (say a flower-pot) to be at hand, filled 

 with nutritious earth, with which we mix our unknown particles, and 

 that, in forty-eight hours subsequently, buds and blades of well- 

 defined cresses and grasses appear above the soil. Suppose the 

 experiment, when repeated over and over again, to yield the same 

 unvarying result. What would be our conclusion ? Should we 

 regard those living plants as the products of dead dust or mineral 

 particles, or should we regard them as the offspring of living seeds ? 

 The reply is unavoidable. We should undoubtedly consider the ex- 

 periment in the flower-pot as clearing up onr pre-existing ignorance. 

 We should regard the fact of their producing cresses and grasses as 

 proof positive that the particles sown in the earth of the pot were the 



