340 SCIENTIFIC RECREATIONS. 



200 ; while oxygen is O, and its weight is 16. Thus we see at once 

 how much of each element is contained in oxide of mercury, and this pro- 

 portion never varies ; there must be 200 of one and 16 of the other, by 

 weight, to produce the oxide. So if the oxygen has to be separated from it, 

 the sum of 216 parts must be taken to procure the 16 parts of oxygen. 

 When we see, as above, O 2 or Og, we know that the weight must be calculated 

 twice or three times, O being 16 ; O 2 is therefore 32 parts by weight. 

 So when we have found what the compounds consist of, we can write 

 them symbolically with ease. 



COMPOSITION OF THE ATMOSPHERIC AIR. 



We have already communicated a variety of facts concerning the air- 

 We have seen that it possesses pressure and weight. We call the gaseous 

 envelope of the earth the atmosphere, and we are justified in concluding that 

 other planets possess an atmosphere also, though of a different nature to ours. 

 We have seen how easy it is to weigh the air, but we may repeat the experi- 

 ment. (See illustration, fig.45, page 50.) We shall find that a perfectly empty 

 glass globe will balance the weights in the scale-pan ; admit the air, and the 

 glass globe will sink. So air possesses weight. We have mentioned the 

 Magdeburg hemispheres, the barometer, the air-pump, and the height and 

 the pressure of the atmosphere have been indicated. The density of the 

 atmosphere decreases as we ascend ; for the first seven miles the density 

 diminishes one-fourth that of the air at the sea-level, and so on for every 

 succeeding seven. 



In consequence of the equal, if enormous, pressure exercised in every 

 direction, we do not perceive the inconvenience, but if the air were removed 

 from inside of a drum, the parchment would quickly collapse. We feel the 

 air when we move rapidly. We breathe the air, and that statement brings 

 us to consider the composition of the atmosphere, which, chemically speaking, 

 may vary a little (as compared with the whole mass) in consequence of changes 

 which are continually taking place, but to all intents and purposes the air is 

 composed as follows, in 100 parts: 



Nitrogen . . 79 parts. 



Oxygen . . 20 



Carbonic Acid. . . '04 



with minute quantities of other ingredients, such as ammonia, iodine, 

 carbonetted hydrogen, hydrochloric acid, sulphuretted hydrogen, nitric acid, 

 carbonic oxide, and dust particles, as visible in the sunbeams, added. 



The true composition of the atmosphere was not known till Lavoisier 

 demonstrated that it consisted of two gases, one of which was the vital fluid, 

 or oxygen, discovered by Priestley. To the other gas Lavoisier gave the name 

 of Azote, an enemy of life, because it caused death if inhaled alone. The 

 carbonic acid in the air varies very much, and in close, heated, and crowded 

 rooms increases to a large quantity, which causes lassitude and headache. 



