DUST AND DISEASE. 7 



most deadly to man. And what is this portion ? It 

 was some time ago the current belief that epidemic 

 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 by which 

 itself had been assailed. Such a power, it was alleged, 

 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 within 

 the human body act in a similar way? In 1836 a 

 very unexpected 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. Here we have active life instead of 

 motor-decay. By this discovery fermentation was con- 

 nected with organic growth. 



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

 dependently 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 supplied solely with calcined air, 

 putrefaction never sets in. Putrefaction, therefore, he 

 affirmed to be caused, not by the air, but by something in 

 the air which could be destroyed by a sufficiently high 

 temperature. The results of Schwann 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, 

 2 



