240 2<> 



which the dust is suspended to appear as a semi-solid 

 rather than as a gas. Nobody could, in the first in- 

 stance, without repugnance place the mouth at the 

 illuminated focus of the electric beam and inhale the 

 dirt revealed there. Nor is the disgust abolished by the 

 reflection that, although we do not see the nastiness, we 

 are churning it in our lungs every hour and minute of 

 our lives. There is no respite to this contact with dirt ; 

 and the wonder is, not that we should from time to time 

 suffer from its presence, but that so small a portion of 

 it would appear to be deadly to man. 



And what is this portion ? It was some time ago the 

 current belief that epidemic diseases generally were pro- 

 pagated by a kind of malaria, which consisted of or- 

 ganic matter in a state of motor-decay ; that when such 

 matter was taken into the body through the lungs or 

 skin, it had the power of spreading there the destroying 

 process which had attacked itself. Such a spreading 

 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 being 

 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 organism, which, when placed in a proper 

 medium, feeds, grows, and reproduces itself, and in this 

 way carries on the process which we name fermentation. 

 Fermentation was thus proved to be a product of life 

 instead of a process of decay. 



Schwann, of Berlin, discovered the yeast plant inde- 

 pendently, and in February, 1837, he also announced the 



