174 Prof. J. Tyndall on the [Jan. 13, 



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 finer matter is extinguished. The coarser motes, on the 

 other hand, flash out with greater force, because of the increased dark- 

 ness of the space around them. It is among the finest ultra-microscopic 

 particles that the author shows that the matter potential as regards the 

 development of Bacterial life is to be sought. 



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 range of the microscope, but within range of the unaided 

 senses. Let it be assumed that our knowledge of them under these cir- 

 cumstances 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 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 

 " spontaneously generated." 



This reasoning applies word for word to the development of Bacteria 

 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 profession. 



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, exposed to 

 filtered air, to calcined air, or shut off altogether from contact with air, 

 are more or less prone to swarm with Bacteria and Vibriones in the 

 course of from two to six days." Who the " we" are who possess this 



