INVISIBLE DOMESTIC SERVANTS 51 



more numerous and more important than we are accus- 

 tomed to suppose. Wherever there is a perceptible current 

 of air, microscopic organisms are wafted with the dust ; 

 wherever the motes dance in the sunbeam, some of them 

 are living motes. A curious variety of minute objects is 

 revealed when the dust of a living-room is spread out on 

 a glass slip. Some little thought must be given in order 

 to discover the neatest way of doing this. It will not do 

 to tease out a visible pellet of dust ; the particles will be 

 found heaped together in utter confusion, and the smallest 

 will escape notice altogether. But if a glass slip, smeared 

 with glycerine, is left undisturbed on a shelf for a day or 

 two, the particles will settle on it one by one, and will be 

 conveniently spaced. Among them an experienced eye 

 will soon detect wood-fibres, hairs of man and sheep, 

 pollen-grains, fern-spores, and still more minute rounded 

 bodies, which are the spores of the common moulds. 

 Closer examination and higher powers are required for the 

 identification of the smallest of all the particles, which 

 are microbes, objects as much smaller than the grains of 

 toothpowder as these are smaller than cherries. Though 

 the microbes are so incredibly minute as to tax severely 

 the resources of the microscopical observer, they are 

 generally still alive when they float in the air of the room, 

 and when they get into a place that suits them, they 

 often show their powers of multiplication in a very sur- 

 prising way. 



When the common moulds establish themselves on a 

 substance that supplies them with food, they often grow 

 so big as to become visible to the eye, and then a pocket- 

 lens will suffice to reveal many interesting details. The 

 moulds spread through the food-substance as branching 

 threads, and only in a comparatively late stage of their 

 life-history send up columns into the air, which form the 

 velvety masses seen on jam, decayed oranges, and old 

 boots. These columns may support capsules, or branch- 

 ing chains, or rays standing out on all sides from a central 



