651 



SPONTANEOUS GENERATION 



test. He argued that if the infusorial animal- 

 cules came from germs, then the germs must 

 exist either in the substance infused or in the 

 water used to make the infusion or in the air that 

 touches both of them. The life of all germs is 

 de-t roved by heat. If, therefore, the infusion be 

 tailed, any germs present will be killed ; and then 

 if the infusion be shut off from the air no more 

 germs can get into it. Now, if after this treat- 

 ment animalcules still appear in the infusion they 

 will have been generated from the infused sub- 

 stance or from the water ; but if they do not 

 appear, then Redi's dogma will be tme for infu- 

 soria. He therefore boiled and corked infusions of 

 various substances, and in every case after a longer 

 or shorter time animalcules appeared and flourished. 

 Needham was associated in much of his work with 

 Button. The French naturalist had a theory of 

 life to which Needham 's experiments lent support. 

 Life, he thought, was the indestructible property 

 of certain molecules, which he described as 'organic 

 molecules.' All living organisms, he said, are 

 built up of such molecules ; death is nothing more 

 than their dissociation. When they are thus set 

 free they take the form of infusorial animalcules. 

 It is necessary to distinguish this theory, which is 

 so ingenious, from the theory of Ahiogenesis, the 

 antithesis to that of Biogenesis, which supposes 

 that life may and does arise from non-living 

 matter. 



But the theory of Buffon, and especially the 

 experiments of Needham, which lent it support, 

 did not seem satisfactory to Spallanzani. He saw 

 two sources of error first, the germs present in 

 the infusion might have escaped death through not 

 having been boded long enough ; and secondly, the 

 corks, perhaps, were not perfectly effective, and air 

 containing germs might have got into the flask 

 and infected the infusion. So he too prepared 

 infusions ; but he boiled them for three-quarters of 

 an hour, and then fused the necks of the flasks. He 

 found after this treatment that the infusions 

 remained perfectly free from living organisms for 

 as long as lie chose to keep them. It might seem 

 that this must be the end of the whole matter ; but 

 the event proved otherwise. This time it was the 

 chemists who reopened the discussion. Oxygen was 

 di-i'overed, the theory of respiration was begun, and 

 it was proved that a supply of free oxygen is one of 

 the conditions of life and also of put refaction. So it 

 was ]H>ssilile that Spallanzani's infusions did not 

 pnn I nee life either liecause the 'organic molecules' 

 were altered in some way by being boiled, or 

 because they were unable, owing to the absence 

 of oxygen, to live. So the experiments had to IK> 

 re|>eated in such a manner that the-organic matter 

 was not altered, and so that there was stiflicient 

 oxygen. Sclmlze and Kchwann in 1836 took up the 

 matter. They carefully boiled their infusions, and 

 then supplied air ; but they made it pass through 

 red-hot tubes first, so that any germs present in it 

 would lie burned. In these conditions no animalcule 

 appeared in the infusions ; but if they were exposed 

 to air which hail not been heated then animalculce 

 npjieared in abundance. Therefore boiling does 

 not injure the vitality of the 'organic molecules,' 

 if there are such, and there is only one possible 

 objection to the conclusions drawn from such experi- 

 mental, if they !> properly conducted, and that is 

 that what the red-hot tnlx's destroy is not germs, 

 but something else that may be non-living and yet 

 essential to life. Now about this time Cagniard 

 de la Tour discovered that fermentation, like putre- 

 faction, is always accompanied by the presence of 

 minute living creatures. Common yeast, for 

 (instance, is a mass of minute plants. When it was 

 (suggested that the living creatures not only accom- 

 panied but actually caused the processes of fer- 



mentation and putrefaction, the chemiatx, led by 

 Liebig and Herzelius, laughed the idea to scorn. 

 Hut in 1843 Helinholtz ingeniously separated a 

 putrescent from a putrescible fluid by a iiiemln. 

 so that the producte of putrefaction could mix with 

 the putrcscihle matter; but that did not in con 

 sequence putrefv. Therefore it followed that the 

 cause of putrefaction must be either a colloid 

 indiU'usilile stuff or a solid. In 1854-59 Scliroeder 

 and Dusch cleared up this point by exjieriments 

 which were simply refinements upon the original 

 ones of Redi ; instead of using a screen of gauze to 

 keep oil' blow Hies they used a screen of cotton-wool, 

 a screen with meshes so fine that not even the tiny 

 germscan pass through them. They lioiled infusions. 

 and while the steam was coming off freely they 

 plugged the neck of the flask with cotton-Wool 

 Now this plug did not keep away oxygen, nor did 

 it in any way heat or alter the air that passed to 

 the fluid, as the red-hot tubes of Schulze and 

 Schwann had done, and yet no animalcules appeared 

 in the boiled infusion screened by cotton-wool. It 

 is therefore proved that the cause of putrefaction 

 and fermentation and the origin of the living forms 

 that accompany these processes must be small 

 particles that exist in the air. 



But in 1859 Pouchet published his Hftfrogtnie. 

 He once more raised clouds of doubt. It seemed 

 evident to him that spontaneous- generation was 

 one of the means that nature employed for 

 the reproduction of living beings. If, he said, 

 all putrefaction is the result of life present, 

 as germs, in the air, then the air in which 

 we live would have almost the density of iron ! 

 About thus time Pasteur took the matter up, 

 although advised by his friends, in view of the 

 difficulty of the subject, not to do so ; at least, 

 said Dumas, do not spend too much time over it. 

 But in 1862 he published a paper On tl<c Onfitnittd 

 Particles existing in the Atmosj>/iere. lie had 

 collected the floating dust of the air and examined 

 it with a microscope. He saw that much of it 

 actually consisted of organised particles, and on his 

 sowing these in suitable sterilised infusions there 

 grew from them rich crops of animalculne. He 

 also showed that these perms of life were not uni- 

 versally diffused in the air. He opened his sealed 

 flasks in the pure air of the Mcr de Glace. Only 

 one out of twenty such flasks thus opened became 

 filled with life ; while eight out of twenty opened 

 in the air of the plains did so, and all of them did 

 so if opened in the air of towns. These researches 

 of Pasteur were necessary before Lister could 

 have brought his system of antiseptic surgery 

 to a successful issue ; while he himself, as is 

 well known, lias done great work upon the re- 

 lations of these forms of life to many industries 

 and diseases. The germs of the air were then 

 studied by Cohn, shown by him to be bacteria, 

 and the basis of a sound knowledge of them was 

 laid. In 1869 T\ ndall hit upon a very precise 

 method of determining the absence or presence of 

 dust particles in the air, a method much more 

 searching than that furnished by the most powerful 

 inicroHco]>e. He was experimenting in another 

 direction, and had need of air free from dust. He 

 noticed that such matter (dust) passed easily 

 through liquids. A beam of light shows the pics 

 ence of dust particles in the air by the reflection 

 that occurs from the surfaces of such. Tyndall 

 showed that whenever dust was present the putre- 

 fact ion occurred sooner or later, when it was absent 

 it did not. Tyndall 's apparatus was a box with 

 glas sides, into the bottom of which the mouths of 

 flasks penetrated, the lower parts being outside tho 

 I" >\. so that the contained infusions could be boiled. 

 A beam of sunlight was allowed to pass through 

 the box ; it showed the presence of dust in the air 



