162 Professor G. V. Poore [April 24, 



autumn I found that the temperature of such a heap had risen in the 

 course of a week or so to 104° F., and remained at a temperature 

 considerably above that of the surrounding air during the whole 

 winter. On turning it over after a month or so one found in it a 

 large number of earth worms and endless fungoid growths visible to 

 the naked eye, and one felt sure that it was swarming with countless 

 millions of bacteria, invisible except to the highest powers of the 

 microscope. In the beginning of March this heap, much reduced in 

 size, was spread loosely over a patch of ground which was previously 

 dug. If one examined that ground to-day one would scarcely recog- 

 nise the structure of leaves, and in a few weeks more it will have 

 become nothing but ordinary garden mould, and anything planted in 

 it will grow with vigour. This is a familiar every-day fact. 



We know also that noisome filth spread over a field by the farmer 

 in the autumn or winter loses its ofiensiveness in a few days, and by 

 the spring neither our eyes or noses give us any clue to the cause of 

 the fertility of the field which is covered with ordinary "mould." 

 This process of " humification " is largely due to earth worms 

 and other earth dwellers, which pass the earth repeatedly through 

 their bodies, and in doing so reduce it to a very fine powder. I have 

 upon the table some worm castings picked off a lawn, and which, 

 after being slowly dried, have been gently sifted through muslin. 

 Those who have never examined a w^orm casting in this way will be 

 interested to see of what an imjmlpable dust the greater part is com- 

 posed, and will also note the considerable size of the pieces of flint 

 and grit which the animal has used in its living mill, and which have 

 been separated by the muslin sieve. 



These castings are full of microbes, and those who will take the 

 trouble to scatter the smallest conceivable j^inch of this impalpal)le 

 dust upon a sterilised potato, after the manner and with all the pre- 

 cautions familiar to bacteriologists, will obtain an abundant and varied 

 growth of bacteria and moulds, which will completely baffle their 

 powers of enumeration and discrimination. 



The greatest hindrance in the bacterial examination of the soil is 

 this p.nibarras de richesses, which makes the isolation of difierent 

 species a matter of extreme difficulty. 



The bacteria exist in the soil in countless millions, but it must 

 be remembered that they get fewer as we go deeper. The first few 

 inches of the soil are, in the matter of bacterial richness, worth all 

 the rest, and at a depth of five or six feet they apj^ear to be almost 

 non-existent. The practical lesson which we have to lay to heart in 

 applying this knowledge is that the upper layers of the soil are the 

 potent layers in bringing about the circulation of organic matters, 

 and that if we wish to hasten this process we must be careful to place 

 our organic refuse near the surface and not to bury it deeply, a pro- 

 cess by which the circulation is inevitably delayed or practically 

 prevented. If we bury it deeply we not only get no good, but we 

 may get harm by poisoning our wells and springs. 



