202 PLANT LIFE. 



to destroy cholera vibrios, or at any rate to make them 

 harmless. 



They are easily killed by many kinds of poison. 

 Thus a solution of 3 5 per cent, alcohol will kill off all 

 bacteria which are likely to be found on glass vessels or 

 upon instruments. A very small percentage of corrosive 

 sublimate is also a bactericide of great value. The oils 

 of Eucalyptus and Cinnamon, and the active principles of 

 Thyme, Turpentine, Lemon, Lavender, and similar sub- 

 stances, are deadly to bacteria. Air filled with the 

 vapour of oil of Lavender has killed the typhus bacillus 

 in twelve hours, and the vapour of oil of cinnamon is 

 even stronger ; for when it was used the bacteria were 

 destroyed in forty-five minutes. Such perfumes and 

 spices are in all probability formed by plants for 

 defensive purposes. In the case of many antiseptic 

 preparations man simply uses for his own ends the 

 protection devised by plants for themselves. 



The functions of bacteria are so varied that it is 

 difficult to classify their work in any convenient way. 

 We will begin however with those found in the soil, and, 

 after mentioning a few which are of great agricultural 

 importance, describe those which produce infectious 

 diseases and are utilised in the disposal of refuse 

 matter. 



Of the Soil Bacteria, one group absorb the free nitrogen 

 in the air and turn it into nitrates which can, of course, 

 enter plants and be used in building up vegetable tissues. 

 In one experiment, two sets of vessels were filled with 

 earth which had been sterilised, or rendered free from 

 Bacteria. Cabbage plants were grown in each set, but one 

 series was inoculated with these soil bacteria, and the 

 other was not infected. 



In every case the Cabbages were heavier and had 



