690 THE POPULAR SCIENCE MONTHLY. 



come, not only within 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 un- 

 known 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, of min- 

 eral particles ; or should we regard them as the ofFspi'ing of living 

 seeds ? The reply is unavoidable. We should undoubtedly consider 

 the experiment with the flower-pot as clearing up our preexisting 

 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 seeds of the plants which have grown from them. It 

 would be simply monstrous to conclude that they had been " sponta- 

 neously generated." 



This reasoning applies word for word to the development of bacte- 

 ria from that floating matter which the electric beam reveals in the 

 air, and in the absence of which no bacterial life has been generated. 

 There seems no flaw in this reasoning ; and it is so simple as to render 

 it unlikely that the notion of bacterial life developed from dead dust 

 can ever gain currency among the members of a great scientific pro- 

 fession. 



A novel mode of experiment has been here pursued, and it may be 

 urged that the conditions laid down by other investigators in this field, 

 which have led to different results, have not been strictly adhered to. 

 To secure accuracy in relation to these differences, the latest words of 

 a writer on this question, who has materially influenced medical thought 

 both in this country and in America, are quoted. " We know," he 

 says, " that boiled turnip or hay infusions exposed to ordinary air, ex- 

 posed to filtered air, to calcined air, or shut off" altogether from contact 

 with air, are more or less prone to sv/arm with bacteria and vibriones 

 in the course of from two to six days." Who the " we " are who pos- 

 sess this knowledge is not stated. Prof. Tyndall is certainly not 

 among the number, though be has sought anxiously for knowledge 

 of the kind. He thus tests the statements in succession. 



And, first, with regard to filtered air. A group of twelve large 

 test-tubes was passed air-tight through a slab of wood coated with 

 cement, in which, while hot, a heated " propagating glass," resembling 

 a large bell-jar, was imbedded. The air within the jar was pumped 

 out several times, air filtered through a plug of cotton-wool being 

 permitted to supply its place. The test-tubes contained infusions of 

 hay, turnip, beef, and mutton, three of each, twelve in all. They are 



