( 73) 



been shaken up in sterile tap-water or in malate solutions, of which 

 1 cm 3 was spread over the plate, care being taken that the water 

 was quite taken up into the agar by its power of imbibition, which 

 is easily effected by softly heating the plate so that the superfluous 

 water evaporates. 



We see from these data that commonly only a small part of the 

 sown germs conies to growth. Whether perhaps the water itself has 

 a deadly influence on some individuals, or that their death 

 is caused by their passing on the solid medium, could not yet be 

 made out by experiment. Thus, although there be ground to allow 

 that more germs occur in the soil used than are found, the possibi- 

 lity exists that by continued investigation the experiment may be 

 made so as to exclude that source of error. 



But in spite of the uncertainty of the method the following result 

 could be stated. By sowing a small quantity, for instance less than 



— gram of garden soil on calciummalate-kaliumphosphate l°/ agar, 



after 24 hours at 3CT C. commonly no Azotobacter is observed, but 

 a moderate number of moist colonies of about 1 mm. in diameter, 

 first draw attention by their extension and prove to consist of 

 different varieties of Bacillus megatherium, containing many spores. 

 They dont cause any considerable oxidation of the malate and as 

 the colonies no more grow after the second day, they evidently 

 develop at the expense of the traces of nitrogen compounds which 

 at first are present in the plates. After the second day a great 

 number of Streptothrix alba appear. This microbe is so common in 

 all the examined samples of soil that there can exist no doubt as to 

 its either favourable or pernicious influence on the fertility ; but 

 the nature of this influence is as yet wholly unknown. 



In a still later stadium the surface of the plate becomes covered 

 with numerous relatively small colonies of bacteria, among which 

 some species immediately draw attention by their extension and 

 commonness. 



The oxidation of the malate by all these microbes is slight, so that 

 even after weeks the plates contain but little calcium carbonate, 

 which seems almost entirely produced by the said larger colonies 

 and by Streptothrix. All these species seem not to oxidise at all, or 

 perhaps it is more accurate to say, not to oxidise any more after the 

 last traces of fixed nitrogen have been assimilated. As to Streptothriv, 

 from its relatively vigorous oxidising power it follows by no means 

 that this should be associated with fixation of nitrogen ; this species 

 surely does not possess that faculty. If for the experiment soil is 



