536 THE POPULAR SCIENCE MONTHLY, 



The drier the air, the more rapidly are the liquids of the body evapo- 

 rated, and digestion and assimilation carried on, the more nervous is the 

 temperament, and the more rapid the development. Generally speak- 

 ing, the air is much drier in the United States than in Europe. This 

 is the chief reason why our children are less repressible, livelier, and 

 more nervous and precocious than those of Europe. Another reason is, 

 that we use here more animal food, which is far more stimulating both 

 to body and mind than vegetable. On the other hand, too dry an at- 

 mosphere is unhealthy. As children drink much water, they exhale 

 much aqueous vapor the sweat-glands and capillary circulation being 

 more active than in the adult say to the amount of half a pint each, 

 more or less, during school-hours. As such a large amount of invisible 

 vapor arises, it serves a useful purpose by adding to the moisture of 

 our dry air, rather than being injurious. In dwellings it is sometimes 

 customary to place a vessel of water upon the stove to produce vapor, 

 so as to diminish the dryness of the air ; but, for the reason given 

 above, it is perhaps unnecessary in a school-room. However, as water 

 absorbs equal volumes of carbonic-acid gas, and four hundred and 

 thirty volumes of ammonia, a shallow vessel of water may in this way 

 be of some service. 



The inorganic matters consist of chalk-dust, earth-dust, ashes, etc. 

 Of late years, owing to the large amount of blackboard work done in 

 schools, particularly in the primary departments, chalk-dust floats in 

 large quantities in the air whenever the erasers are used. The parti- 

 cles of chalk-dust are comparatively large in size. When inhaled, it 

 lodges in the posterior portion of the nasal passages and upper portion 

 of the larynx ; and when settled in large amount in these locations it 

 gives rise to a good deal of irritation. The effect of this irritation is 

 the secretion of a tenacious mucus that provokes distressing cough and 

 unpleasant hawking. It is easy to understand how this exciting cause, 

 long continued, may produce a chronic catarrh of these regions, espe- 

 cially in the posterior nasal passages, as they are prone to congestion 

 and a low grade of chronic inflammation. The same remarks apply, 

 but in a far less degree, to ash and earth-dust. The frequent cough 

 and occasional sneeze heard among the audience in theatre, hall, or 

 church, are provoked by the inhalation of fine dust suspended in the 

 air, and might be prevented by careful sweeping and dusting after 

 occupancy. The school-room should be swept every evening, and 

 dusted at least an hour before opening. The blackboards should be 

 erased as little as possible, and preferably by the so-called " dustless " 

 erasers though, strictly speaking, no eraser is really " dustless," being 

 simply "less dusty" and then gently in an up-and-down direction, so 

 that the dust may not be dispersed through the room. The floor 

 should not be disturbed by sweeping at any time during the day. 

 Having examined briefly the different substances that vitiate or foul 

 the air contained in a school-room, and the sources from which they 



