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 
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 
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 
Kinds of 
Soil 
Souring 
of Milk 
