I! 



484 ON THE CAUSATION OF 



structures, such as pieces of hair or vegetable fibre ; and if these are suspended 

 in the air, stih more must microscopic spores be so, though their extreme 

 minuteness makes it less easy to distinguish them from particles of inorganic 

 matter. Hence it appears that, for the lowest forms of life, as for the highest 

 the notion of spontaneous generation is simply gratuitous and uncalled for. 



But although from these considerations we may be led pretty surely to 

 infer, on the one hand, that the atmosphere is pervaded by the germs of minute 

 organisms, and, on the other hand, that without such germs the organisms 

 could not take their origin, it would be highly desirable to obtain positive 

 evidence on both these points, if indeed it is attainable. 



Such evidence has been afforded of late years by the beautiful researches 

 of Pasteur. From among his numerous experiments, I will select one set as 

 peculiarly instructive. A number of glass flasks, with attenuated necks, were 

 partially filled with a decoction of yeast, filtered so as to be perfectly clear 

 and transparent. Each was then boiled for a certain length of time, with 

 the object of destroying any organisms existing in the decoction, or adhering 

 to the interior of the vessel, and during ebullition the neck was hermetically 

 sealed, so that when the vessel cooled, a vacuum was produced in the part 

 previously occupied by air. A certain number of such a series of flasks were 

 then opened in a particular locality, as, for example, a lecture-room such as 

 this, by breaking the narrow neck of each, after scratching it with a file. 

 Air rushed in to fill the vacuum, after which the neck was immediately sealed 

 again with the blow-pipe. As the result of the introduction of this limited 

 amount of air, the previously transparent liquid in a considerable proportion 

 of the flasks was seen to present, in the course of the next few days, a cloudi- 

 ness indicative of the first appearance of the growth of torulae and other 

 organisms, which afterwards continued to increase. But if a set of such flasks 

 were opened in a situation where atmospheric germs might be expected to 

 be few, if any, a different result was obtained. M. Pasteur was at the pains 

 to take such flasks to the Mont Anvert, in Switzerland, and open them in 

 wind blowing from a glacier, taking special care, by exposing the neck to the 

 flame of a spirit-lamp when filing it, and breaking it with long forceps similarly 

 treated, to guard as much as possible against the introduction of living 

 organisms from the instruments employed, or from his own person. The pure | 



air thus introduced had indeed, in one flask out of twenty, the effect of inducing, 

 very slowly, an appearance of organic development. But in all the rest the 

 liquid remained perfectly unchanged for an indefinite period. On the other 

 hand, if the flasks were opened in a situation where the air, though in one 

 sense pure, might be expected to abound in minute life, viz. under the shade 



