18 THE WORLD'S LUMBER JKoo.tr. 



need only leave a basin of paste in their way, and we 

 shall often find it overgrown with " mould " in a single 

 night. 



Far lower again in the scale of life than either " mould " 

 or mildew, are the various organisms bearing the general 

 name of " bacteria " (Fig. 8), which are the agents of all 



putrefaction and fer- 

 \ l '* / ^ / mentation, and the 



, '' V / . > o'* 3 ^ - ,. 



a- f *^-*. * T V. SN b cause of many diseases. 



x frvAr** - These swarm in all 



4 i* 



c moist places of the 



earth, and are wafted 

 into the air in immense 

 Fig. 8. BACTERIA : a, 600 times their natural numbers 

 size ; b, 1,000 times natural size ; c , Bac- 

 teriurn, 5,000 times natural size. oUCh, then, IS the 



floating matter of 



the air, and we must now see how its absence would 

 affect us. 



We have said that but for the floating dust we should 

 probably have no mist, or fog, or clouds. 



Steam, or water-gas, is, like other gases, invisible ; but, 

 as it escapes through the spout of the kettle the cooler 

 air of the room makes it condense into a cloud, consisting 

 of numberless minute globules of water. But the air of 

 the room, besides being cool, is also dusty ; and if the 

 steam be received into a glass vessel containing only per- 

 fectly-filtered air, no cloud at all will be formed, but the 

 steam will condense into dewdrops upon the glass. 



Hence, from repeated experiments, it seems that when- 

 ever water-gas condenses into mist or fog, each globule 



