286 FRAGMENTS OF SCIENCE. 



diseases generally were propagated by a kind of malaria, 

 which consisted of organic matter in a state of motor-decay; 

 that when such matter was taken into the body through 

 the lungs, skin, or stomach, it had the power of spreading 

 there the destroying process which had attacked itself. 

 Such a power was visibly exerted in the case of yeast. A 

 little leaven was seen to leaven the whole lump, a mere 

 speck of matter in this supposed state of decomposition be- 

 ing apparently competent to propagate indefinitely its own 

 decay. Why should not a bit of rotten malaria work in a 

 similar manner within the human frame ? In 1836 a very 

 wonderful reply was given to this question. In that year 

 Cagniard de la Tour discovered the yeast-plant, a living or- 

 ganism, which, when placed in a proper medium, feeds, grows, 

 and reproduces itself, and in this way carries on the process 

 which we name fermentation. By this striking discovery 

 fermentation was connected with organic growth. 



Schwann, of Berlin, discovered the yeast-plant inde- 

 pendently about the same time ; and in February, 1837, he 

 also announced the important result that, when a decoction 

 of meat is effectually screened from ordinary air, and sup- 

 plied solely with calcined air, putrefaction never sets in. 

 Putrefaction, therefore, he affirmed to be caused by some- 

 thing derived from the air, which something could be de- 

 stroyed by a sufficiently high temperature. The results of 

 Swann were confirmed by the independent experiments of 

 Helmholtz, Ure, and Pasteur, while other methods, pursued 

 by Schultze and by Schroeder and Dusch, led to the same 

 result. But as regards fermentation, the minds of chemists, 

 influenced probably by the great authority of Gay-Lussac, 

 fell back upon the old notion of matter in a state of decay. 

 It was not the living yeast-plant, but the dead or dying 

 parts of it, which, assailed by oxygen, produced the fer- 

 mentation. This notion was finally exploded by Pasteur. 

 He proved that the so-called " ferments " are not such ; 



