198 NITROGEN. 



oxygen. The process of analysis of the air is called eudiometry, 

 the instrument, an eudiometer.* 



5. CONDITION OF THE ELEMENTS OF THE ATMOSPHERE. 

 (a.) It has been already stated, that most chemists suppose the at- 

 mosphere to be a mixture of the two gases. In favor of this view, it 

 may be said that there is not, as in most cases of chemical combi- 

 nation, any change in volume ; 4 volumes of nitrogen and 1 of oxy- 

 gen, forming precisely 5 volumes of the mixture ; the refractive pow- 

 er and the agency in combustion and respiration, is just what would 

 arise from the operation of the mixed gases, and even water, in a de- 

 gree, separates them, because ebullition expels from rain water more 

 than 28 per cent, of oxygen ; j- the extended surface of the drops of 

 rain being peculiarly favorable to the efficiency of a weak affinity ; 

 also, a small quantity of air agitated with a large quantity of water, 

 has all its oxygen absorbed, and but little of its nitrogen. On the 

 other hand, as the proportions, both by volume and weight, corres- 

 pond with the theory of definite proportions ; as there is no inequality 

 in the mixture arising from the difference in specific gravity, the at- 

 mosphere being every where the same ;{ even if the gases are not com- 

 bined, the winds would tend greatly to preserve, in equable mixture, 

 aeriform fluids whose gravity is so nearly equal. 



(b.) Perhaps it is, rather, a feeble combination. Analogous to the 

 many which exist between palpable substances where the properties 

 are not altered. (See p. 159.) There is no improbability that gases 

 may be united by a very feeble affinity, and a strong one would, in 

 this case, be incompatible with the exigencies of animal and vegeta- 

 ble life, and with the demands of combustion. It is indispensable 

 that the atmosphere yield up its elements readily. 



6. Constancy of the proportions. 



(a.) They never vary, except from the operation of limited local 

 causes, such as combustion and respiration. The air which Gay 

 Lussac brought down from 21.735 feet above the earth, contained 



* The term alludes to the health of the atmosphere, as it was supposed to be af- 

 fected by the proportion of oxygen ; the Greek particle i>, signifying well, and Atoj, 

 the atmosphere, derived from Jupiter, which in Greek is Zev$, Gen. Awj, used for 

 the atmosphere, t Edin. Jour. No. 8, p. 211, quoted by Dr. Turner. 



t Mr. Dalton's views of the constitution of the atmosphere and of mixed gases, are 

 opposed to this opinion. See Henry, Vol. I, p. 299, 10th Lon. Ed. In a vertical 

 tube, or in two vials thus connected by a tube, hydrogen gas will in a few hours de- 

 scend, and carbonic acid gas ascend, so as to mix with each other contrary to 

 gravity. Still, in chemical experiments, we find it important to favor the mixing of 

 gases of remarkably different specific gravity, by adding the lightest, last; otherwise 

 the mixture will be imperfect and tardy. The great mobility of gases, and the waves 

 and currents so easily produced in them by even slight variations of temperature, 

 might be expected to favor their mixture in the course of time. 



At that height, an exhausted bottle was opened, filled with air, and then closed ; 

 after his descent, it was opened under water, which rushed in and filled half of it, 

 thus proving the great rarity of the air. 



