THE STORY OF THE LUNGS, SKIN AND KIDNEYS 79 



to more than four parts in 10,000 of air, the purity 

 of the air may be taken for granted, that is, having 

 regard to its being safe for breathing. When on the 

 other hand the quantity of carbonic acid gas mounts 

 up, as it does in close and confined places, the air then 

 becomes deleterious to health. In ordinary air a vari- 

 able amount of watery vapour is always found, and in 

 very pure air, especially that of the mountain and 

 the sea, ozone occurs, this last being a very active 

 form of oxygen presumed to have stimulating effects 

 on the body, and therefore credited with giving to 

 mountain air and that of the ocean, their invigorating 

 properties. The air in addition contains a certain 

 amount of suspended matter in the form of invisible 

 dust. This dust becomes visible under certain cir- 

 cumstances as when a beam of electric light is sent 

 under certain conditions through air, and we can 

 realise the immense amount of floating dust which 

 ordinary air may contain when in a darkened room 

 we see countless motes and beams dancing in the 

 track of a beam of sunlight passing in through a 

 clink in the closed shutters. Much of this dust con- 

 sists of mineral matter, but a certain proportion has 

 to be ranked under the head of organic matter, that 

 is, matter either itself alive or derived from living 

 bodies. The living dust of the atmosphere is repre- 

 sented by germs or microbes of various kinds, many 

 of them harmless, some of them undoubtedly disease- 

 producing. A certain amount also of dead material is 

 represented in the air, consisting of the worn out cells 

 and particles derived from animal and plant bodies. 

 It may be added that from the lungs with each breath 

 there is also expelled, in addition to heat, water and 

 carbonic acid, a certain amount of organic matter 

 representing the worn out tissues and particles 



