1 66 .LEISURE-TIME STUDIES. 



tation are too severe, and your experiments must count for 

 nothing in the balancing of evidence, until you prove the 

 stability of the chemical conditions to which the fluids and 

 the matter thereof have been subjected." Hence the new 

 generation of investigators which succeeded the Italian Abbe, 

 had to assure themselves that the conditions necessary and 

 adapted for the production of life were kept intact and 

 unaffected as regards chemical influences. 



The first experiments of note which were undertaken 

 under these latter auspices were those of two German 

 observers, Schulze and Schwann, who, about 1836 and 1837, 

 conducted some investigations on the fertility of liquids. 

 An infusion, which had been duly boiled, was thus placed 

 in a flask to which atmospheric air could gain admittance ; 

 the air, however, being first made to pass through certain 

 chemical substances. Air was thus literally filtered through 

 glass tubes heated to a high degree, such a condition of 

 great heat being capable of destroying germs, but leaving the 

 oxygen of the air, so necessary for the development of life 

 of all kinds, perfectly unaltered. In other investigations, 

 these experimenters filtered the air through vitriol and 

 caustic potash, two substances well known as destroyers of 

 organic or living matter, but which possess no effect on the 

 oxygen of the atmosphere. The results of these experiments 

 were highly satisfactory to the biogenesists. No traces of life 

 appeared in these protected infusions, and the doubts regard- 

 ing the deprivation of life-bearing conditions which chemists 

 had raised, were thus effectually dispelled. When the protec- 

 tive chemicals were withdrawn, and unfiltered air allowed to 

 gain access to the flasks, a full development of animalcular 

 life appeared; this positive result serving as an important 

 counter-proof to the negative results previously obtained. 



Meanwhile, microscopic science had been making im- 

 portant advances. In 1843 De la Tour discovered the fact 

 that the essential element in yeast is a microscopic plant, 

 and that fermentation must be regarded as a definite result 

 of the growth and propagation of these minute organisms. 



