

PNEUMATICS. 37 



oxygeii in an hour, and consequently destroys the vital 

 principle of five times that quantity, the necessity of 

 proper ventilation will be perceived in order to procure 

 a constant and fresh supply. Air is sometimes rendered 

 very deleterious by vapours escaping from the earth. In 

 Campania the air is impregnated with sulphur, and it has 

 also been found to contain arsenic, and millions of fishes 

 are said to be destroyed by it. At a certain part of 

 Sweden, noted for its copper-mines, the air is so affected 

 by the exhalations as to tarnish the silver in the pocket. 



Air becomes expanded by heat and condensed by cold, 

 in the proportion, according to Laplace, of one four- 

 hundred and fiftieth for every degree of Fahrenheit's ther- 

 mometer. To this property may be attributed those cur- 

 rents of air called winds : the heat of the sun expanding 

 the air, and causing it to become lighter, it consequently 

 ascends, while the circumambient air rushes in to supply 

 its place. The following experiments will show the effect 

 of heat in rarefying the air : Hold a bladder partly filled 

 with air to the fire, and the air will become so expanded 

 by the heat as to burst the bladder with a considerable 

 report. Again, take a wine glass and hold it over a piece 

 of lighted paper for about a second, then suddenly clap 

 it firm upon the palm of the hand, the glass will remain 

 tightly fixed on the hand from the inner pressure being 

 diminished through the rarefaction of the air, and the 

 flesh will be pressed into the glass by means of the op- 

 posite pressure. Fire-balloons are made on the principle 

 of the rarefaction of the air by heat. A piece of sponge, 

 soaked either in turpentine or in spirits of wine, is sus- 

 pended by means of a wire at the bottom of the balloon, 

 which when ignited so rarefies the air within the balloon 

 as to cause it to ascend. The first balloons were thus 

 made. M. Montgolfier, a Frenchman, in the year 17<S3, 

 made one of canvass, seventy-four feet high and forty- 

 eight in diameter, weighing in the whole 100O pounds ; 

 and notwithstanding it was of this immense weight, it 

 was filled in ten minutes with rarefied air, when its power 

 of ascension was equal to more than six hundred weight. 



Air has a refractive and reflective power. By its power 

 of refraction the rays of light are bent out of their direct 

 course, as in the case of the twilight j by which means 



