ON DUST AND DISEASE. 137 



so small a portion of it, and even that but rarely 

 diffused over large areas, should appear to be 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 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 pro- 

 pagate indefinitely its own decay. Why should not a 

 bit of rotten malaria act 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. By this striking 

 discovery fermentation was connected 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 

 which could be destroyed by a sufficiently high tem- 

 perature. 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. 



