DUST GARDENS. 13 



sometimes crowd together. Probably these were all 

 attached to some tiny fibre of wood or cloth. 



The soil or "nutrient gelatine" in our experiments Kindg 

 had beef juice in it; you will ask if any other soil 

 would do. The gardener knows that his pinks will 

 grow better in one place and his ferns in another be- 

 cause each requires or likes, we may say, a particular 

 kind of food which that soil contains. 



In the laboratory numerous soils or nutrient media 

 are used milk, potato, beer, blood-serum, etc. 



A moment's thought will show that all the food 

 substances which we like best are subject to changes 

 which in general we designate as "spoiling." Some 

 grow bitter, some sour, some odorous, some rancid. 

 In a few cases this result is due to processes brought 

 about by mere chemical changes that is, without the 

 intervention of any living agent or ferment; but in 

 most cases where food spoils, it is due to the growth 

 on or in the substance itself of the little plants, which 

 have been carried to it through ordinary dust. 



The milk in the pantry is found to be sour. When 

 it was secreted by the milk gland in the cow's body it of UI M 

 was sweet and pure. It passed down into the milk- 

 duct in its passage outward, and here perhaps it met 

 a few of the dust-plants which had passed into the 

 mouth of the duct from the outside. Hundreds, no 

 doubt, fell into the pail from the dusty air of the stall 

 the cow's hairy coat, the milkman's clothes, or hands, 

 or hair, even from the pail itself, for all are more 



