4 POPULAE SCIENCE. 



atmospheric air as pure, but no purer than the atmosphere 

 without, or, at the very utmost, separating the gross fuligi- 

 nous particles due to smoke by a rude contrivance of filtration, 

 as was accomplished by Dr. Eeid in the new Houses of Par- 

 liament. It is impossible after recent discoveries that venti- 

 lation can remain at that point of immature development. 

 When certain facts in the history of atmospheric physical 

 analysis shall have been laid before the reader in something 

 like the order of their succession, the practical bearing of 

 Dr. Tyndall's late illustrations will become apparent. There 

 is a certain class of phenomena' known to experimentalists by 

 the general name of ' catalysis ;' a word of indefinite meaning, 

 almost equivalent to an admission that the so-called catalytic 

 function was not understood at all. Some catalytic agencies 

 refer to the inorganic, others to the -organic, kingdom ; but 

 the time seems almost come, if it is not already come, for 

 abandoning the idea of organic catalysis altogether. 



Not many years ago the change of sugar to alcohol under 

 the action of yeast was said to be a catalytic change ; in other 

 words, that the yeast acted as a ferment by mere contact in 

 some inscrutable way. About the year 1836, however, the 

 discovery was made that the transmutation was an organic 

 change due to the growth of certain minute fungi, none the 

 less real because they were minute. Proceeding from this 

 starting-point, microscopists soon found that whenever any 

 sort of fermentation or decay occurred, myriads of small 

 living beings in some cases vegetable, in others animal, in 

 yet others indeterminate were to be found. Thus, if beef- 

 tea be set aside in the open air, it soon turns sour; and if, 

 when in this condition, it be microscopically examined, it 

 teems with animalcules. Again, the paste of wheaten flour, 

 if set aside for a while in the open air, also becomes the 

 abode of living beings. These are two instances out of 

 many ; the generalisation being, that every fermenting or pu- 

 trefying body is the abode of some form or forms of organic 



