THE AIR OF TOWNS. 351 



25 to 45 years by far the largest number (122) are due to phthisis, and 

 the next largest number (38) to pneumonia. This high percentage of 

 deaths from such diseases is characteristic of all large manufacturiug 

 centers. 



But we need not have recourse to these statistics to assure ourselves 

 of the beneficial effects of fresh air. We have all experienced them. 

 Statistics, however, emphasize the cumulative effect of imperceptible 

 changes — an effect which you will all admit is sufficiently serious. 



There is such a thing known as cumulati ve poisoning. White lead, for 

 example, taken internally in minute quantities will in time produce the 

 effect of a poisonous dose. Bad air is also an example of a cumulative 

 poison. 



According to Professor Foster, the average individual inhales 2,G00 

 gallons of air in twenty-four hours, or about 34 pounds by weight, as 

 against 5£ pounds of food, liquid and solid, or six times the weight of 

 food. If we had to buy our air at so much a pound or pay rates on it 

 at so much a cubic foot or gallon, we should take good care that it was 

 not adulterated; for we distinguish fresh air as we do fresh butter from 

 the second-rate article. There is, however, an important distinction 

 between food and air regarded in this way. If the food we take is not 

 quite as nourishing or as good as it should be, the digestive process is 

 sufficiently adaptable to select the good and reject the bad; but the 

 lungs are infinitely more delicate in structure and function, and we 

 can not with impunity inhale a vitiated air and expect our lungs to 

 select the pure and reject the impure without permanent injury to our 

 breathing apparatus as well as to our whole body. 



Before passing to the subject of "Town air," I should like you to 

 grasp and keep well before you the idea that we are living at the 

 bottom of a great ocean of air, that we are surrounded on all sides 

 by matter invisible because composed of minute particles (separated by 

 spaces which are big in comparison with the particles) but none the 

 less material. 



That the air has weight was first demonstrated by Galileo about the 

 middle of the seventeenth century. I will repeat his experiment : 



A glass globe (fig. 1), furnished with a brass stopcock is evacuated 

 by the air pump, the stopcock closed and the vessel then carefully coun- 

 terpoised. On opening the stopcock air rushes in with a hissing sound, 

 and the balance now sinks at the arm to which the globe is sus- 

 pended, thus showing that the air has weight. 



Now, this invisible matter or gas is not a single gas, but a mixture of 

 gases — mainly two. 



One of these gases is nitrogen, an inert gas, whose chief properties 

 are negative. It constitutes about four-fifths of the total bulk of the 

 air and serves to dilute the other constituent, oxygen, which is the active 

 part. This gas helps things to burn and supports life by consuming 

 waste tissue and keeping up the animal heat. In these processes the 



