88 ELEMENTS OF GENERAL SCIENCE 



fact that a covering of dust soon settles on the furniture 

 which was carefully cleaned but a few hours before is one 

 of common observation. When we go out of doors, dust 

 often gets into our mouths and nostrils, irritates our eyes, 

 and clings to our clothing. Indoors we cannot escape it, for 

 even though we close the house it niters through the smallest 

 cracks. It is upon the food we eat, particularly if we pur- 

 chase articles from street stands or in stores where the food 

 is displayed uncovered. In cities the air is often so filled 

 with solid particles, whether called dust or smoke, that it is 

 impossible to see very far. Dust is not wholly absent even 

 in the clear air over the desert or the ocean. 



100. The sources of dust. What is this dust which we 

 might eat with fruit purchased from a street stand at some 

 dusty corner ? Undoubtedly a large part of it comes from 

 the dust of the street, ground up by the hoofs of horses and 

 the wheels of vehicles. A glance into the street will enable 

 us to see some of the things which are being crashed to 

 rtowder and which will later make part of the common dust 

 as it is picked up by the wind and whirled about. Some 

 particles are bits of unbumed carbon (smoke or soot), and 

 in cities this may make up a large part of the dust Bits of 

 wool and cotton from the clothing of all sorts of people, 

 fragments of hairs from the coats of animals, dead cells from 

 the skins of people and animals all these things and many 

 more enter into the composition of dust. Some of the dust 

 particles may irritate the delicate membranes of the nose 

 and lungs. A larger particle in the eye may occasion serious 

 trouble. But if these and the household inconveniences were 

 all, we might be inclined to think dust merely an annoyance. 



101. Living dust. We shall get some more information 

 about these particles in the air if we will perform certain 

 experiments. Cut a slice from a fresh loaf of bread, using 

 a clean knife. If this slice is exposed to the air for a while 

 and then covered up in such a way that it cannot get dry, 



