14 BIOLOGY 



alive in the material after boiling, i.e., that it was sterile, 

 and to be sure that the sealing was effectual. Two names 

 especially connected with this dispute were Needham, in 1749, 

 and Spallanzani, in 1777. Needham believed firmly in spon- 

 taneous generation, while Spallanzani insisted that the micro- 

 scopic organisms that appeared in these experiments were 

 either there originally and not killed by the boiling to which 

 the material had been subjected, or had found their way into 

 the solutions through microscopic cracks left by the imperfect 

 sealing. 



Pasteur and Appert. In the middle of the last century the 

 French scientist, Pasteur, carried out a series of experiments 

 and attained results which conclusively disproved the theory 

 of spontaneous generation. But the long debated question 

 would not be settled even then. It is a curiously interesting 

 fact that, while scientists were disputing over this matter, the 

 question had, for practical purposes, actually been settled by 

 Appert, who in 1831 had discovered the method of preserving 

 animal and vegetable foods by the means of heat and sealing, 

 the method used by the canning industries of the present day. 

 But the significance of this practical discovery was not appre- 

 ciated, and the dispute continued even after Pasteur's work, 

 the advocates of spontaneous generation continuing as insistent 

 in their claims as ever. The settlement of the question 

 was not reached until the English physicist, Tyndall, devised 

 a new and ingenious method of experimenting which so satis- 

 factorily guarded all sources of error that criticism was silenced. 

 Indeed, so convincing were his experiments that his conclusions 

 have practically never been questioned. 



Tyndall's Experiments. Briefly, Tyndall's method of ex- 

 perimenting was as follows: An airtight box was constructed, 

 rectangular in shape and provided at either end and in front 

 with glass windows. Into the top of this box passed small 

 glass tubes which had been thrown into several curves, through 

 which the air was allowed to enter freely; Fig. 3 a. 



