VI.] THE ATMOSPHERE. 87 



in the present state of our knowledge fermanent gases. 

 Oxygen and nitrogen are examples of gases which are not 

 known in any other than the gaseous condition. The 

 atmosphere is therefore a mixture of the permanent gases — 

 oxygen and nitrogen, with subordinate quantities of the 

 condensible gases — carbonic acid and steam. 



When a liquid is evaporated, or converted into gas or 

 vapour, it undergoes a great increase of bulk, but its weight 

 renains unaffected. A pound of water, for example, pro- 

 duces neither more nor less than a pound of steam. It is 

 cleir, therefore, that gases and vapours, although generally 

 invisible, must possess weight ; but this weight is necessarily 

 small compared with that of the same bulk of matter in 

 the liquid or solid state. Atmospheric air is, in fact, about 

 800 times lighter than an equal bulk of water, and as much 

 as : 1,000 times lighter than an equal volume of quicksilver. 

 Yet the weight of air, small as it seems, amounts to some- 

 thirg considerable when we are dealing with a large bulk, 

 or even with such a quantity as is contained in an ordinary 

 dwelling-room. It is found by actual weighing that 100 

 cubic inches of air, under ordinary conditions, weigh about 

 31 grains ; in other words, it requires 13 cubic feet of air to 

 weigh a pound avoirdupois. Suppose then that we have a 

 room measuring 10 feet long, 10 feet wide, and 10 feet high: 

 this will contain 1,000 cubic feet of air, and the weight of 

 this air will be about 77 pounds. But the room just taken 

 is a very small one, and if the calculation be extended to 

 a large public building it will be found that the air which it 

 contains weighs more than is commonly imagined. Thus, 

 Westminster Hall has a length of 290 feet,, a width of 68 

 feet, and a height of no feet; its contents must therefore 

 be 2,169,200 cubic feet, and the weight of the air in this 

 hall reaches the enormous amount of nearly 75 tens! 



