XII.] BACTERIA. 411 



be carried about still more easily than Torula are. In 

 fact there is reason to believe that they are very widely 

 diffused through the air, and that they exist in abundance 

 in all ordinary water and on the surface of all vessels that 

 are not chemically clean. They may be readily filtered off 

 from the air, however, by causing it to pass through cotton- 

 wool. 



The Bacteria have no near relationship to any of the 

 other plants described in this book, and are introduced 

 chiefly on account of the great importance of the various 

 fermentative changes which they produce. 



The life-history of the Bacteria is only completely known 

 in comparatively few species. Some of them have been 

 found to be highly pleomorphic, that is to say, the same species 

 may appear under very different forms at different periods 

 of its development. In other cases the life-history is com- 

 paratively simple. Hence the various forms mentioned in 

 the Laboratory work must not be regarded as necessarily 

 representing so many distinct species. 



LABORATORY WORK. 



Infuse some hay in warm water for half-an-hour — 

 filter, and set aside the filtrate: note the changes 

 which go on in it — at first clear, in 24 or 36 hours it 

 becomes turbid ; later on, a scum forms on the sur- 

 face and the infusion acquires a putrefactive odour. 

 Rub some gamboge down in water and examine a 

 drop of the mixture with a high power: avoid all 

 currents in the fluid and watch the Brownian move- 

 ments; note that they are simply oscillatory — not 

 translative. 



