80 ELEMENTS OF GENERAL SCIENCE 



It is certainly worth while to find out something about this 

 ever-present dust. We ought at least to know whether it is 

 harmful or helpful. 



89. The nature of dust. What is this dust which we eat 

 along with the apples or other fruit that is 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 

 crushed to powder, and which will later make part of the 

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



Another part comes from greater distances, though it may 

 have had a similar origin. Some particles are bits of unburned 

 carbon (smoke or soot), and in cities this may make up a 

 large part of the whole amount of dust. Bits of wool and 

 cotton from 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 the dust. 



If this were all, it might not seem so bad. True, some of 

 these things offend our sensibilities, and some of them may 

 irritate the delicate membranes of the nose and lungs, or a 

 larger particle in the eye may occasion more serious trouble, 

 but if this and the household inconvenience were all, we 

 might be inclined to think dust merely an annoyance. 



90. Living dust. We shall get some more information about 

 these particles in the air if we will perform certain experi- 

 ments. 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, within a few 

 days it will be covered with a growth of fine, white, thread-like 

 bodies and will exhibit the appearance of the growth to which 

 we apply the name mold. Mold is a living plant. 



Within a few days we shall see that the mold is bearing 

 small black heads which yield a fine, dust-like black powder. 



