THE ATMOSPHERE.] 



CHEMISTRY. 



323 



immediately extinguished. Afterwards, raise the jar, 

 mouth upwards, and try its contents again, after a 

 few seconds, by means of a lighted taper. The gas, 

 owing to its levity, will escape, and the taper will burn 

 as in the atmosphere, because the external air has taken 

 the place of the nitrogen. 



These experiments illustrate the fact that nitrogen 

 does not support combustion ; and that it has, like 

 hydrogen, a less specific gravity than the atmosphere. 



The Atmosphere. The most important use of nitrogen 

 is that found in the air we breathe. Were it composed 

 entirely of pure oxygen, it would be impossible to ex- 

 tinguish a fire which had once been kindled. All animal 

 and vegetable life would be too highly stimulated, and 

 our present state of existence would be rendered im- 

 possible. A hundred cubic inches of air weigh about 

 31 grains ; and as it has been already often stated, dry 

 air at a temperature of 60, and a barometrical column, 

 equal to support 30 inches of mercury, is taken as a 

 standard for the specific gravity of all other gases. The 

 following tables give an approximate statement of the 

 composition of atmospheric air : 



Composition of the Atmosphere, 

 1. By weight 



Oxygen, the active element .... 23 

 Nitrogen, the negative element. . . 77 



100 

 2 By measure 



Oxygen 21 



Nitrogen 79 



100 



But in the above, we suppose that absolutely pine air 

 is estimated : such, however, is never found in nature ; 

 carbonic acid and other gases being always present with 

 aqueous vapour, in variable quantities. 



We cannot enter here into any extended account of 

 the physical character of the atmosphere. We may, 

 however, state, that it is considered to encircle our 

 earth to the extent of 45 miles from its surface ; hence 

 a column of air of that height, one of water 32 to 34 

 feet high, and one of mercury of about 30 inches high, 

 on the average, balance each other. The barometer 

 which will be fully described under the head of Pneu- 

 matics is employed to measure the variations of at- 

 mospheric pressure, which, at the surface of the earth, 

 at the level of the sea, is equal to about 15 Ibs. on the 

 square inch ; diminishing as the height of the atmosphere 

 from the surface increases. Hence, the higher the at- 

 mosphere, the less the pressure it exerts on any surface. 

 The causes of the barometric variations, or of the pres- 

 sure of the atmosphere, are very numerous ; and, until 

 lately, had defied accurate or approximate rules. Owing, 

 however, to extensive observations taken in all parts of 

 the world, the science of meteorology, in which are 

 included such subjects, is rapidly advancing to a high 

 position ; and it is even now possible to foretel, with 

 tolerable accuracy, the occurrence of any extraordinary 

 atmospheric fluctuation. 



It U a remarkable fact, that although we cannot view 

 the MmMphera in the light of a chemical compound, 

 its constitution never varies to any great extent. The 

 air obtained from the top of a mountain, and that on 

 the plains, at first is identical in composition. Local 

 and accidental circumstances cause some variations. 

 Thus, in crowded cities, more carbonic acid and other 

 gates are present than in the open country. Again, 

 after a thunder-storm, small quantities of nitric acid 

 have been detected, that acid being generally formed 

 when the electric spark is sent through air. Sulphuric, 

 sulphurous, and hydrochloric acids, ammonia, common 

 salt, sulphuretted hydrogen, &c. , may also be noticed; 

 but the presence of these depends on causes which are 

 entirely local and controllable. 



There are various methods employed to analyse the 

 air ; but of these we shall have more fully to speak when 

 we consider tho sulijoct of cueoiijiU analysis. Tno 



method we recommended for procuring nitrogen for tha 

 purposes of experiment, illustrates these. The object 

 always is, to remove the oxygen either by combustion or 

 absorption. The former is generally done by means of 

 hydrogen, when water is formed by the combination of 

 that gas, with the oxygen contained in the air undergoing 

 examination. A solution of protosulphate of iron, with 

 a g is we shall presently describe the binoxide of nitrogen 

 is employed when the process of absorption is adopted ; 

 such a solution having great attraction for free oxygen, 

 and thus removing it from the air. 



We need scarcely here dilate on the necessity of pure 

 air for the maintenance of health, as we have already 

 made extended remarks on that subject when speaking 

 of ventilation, under the section of Heat. We may, 

 however, point out the remarkable fact, that gases of 

 very different specific gravities, have the extraordinary 

 power of rapidly diffusing themselves with each other. 

 Thus, if a tube be divided by a thin bladder, or dia- 

 phragm of plaster of Paris, and one end be filled with 

 hydrogen, and the other with carbonic acid gas the 

 latter, which has a great specific gravity, weighing 

 twenty times at least as much as its bulk of hydrogen, 

 will, if placed in the lower end of the tube, gradually 

 rise and diffuse itself with that gas, which again will 

 descend, and equally diffuse itself with the carbonic acid 

 gas. Thus the constitution of the atmosphere, whilst 

 kept uniform, under ordinary circumstances, by this law 

 of gaseous diffusion, is, as we have already intimated, 

 deteriorated by other gases. Hence arise miasma, and 

 those evil results produced on the animal economy by 

 vitiated air and noxious gases. These subjects have 

 been the study of many leading British and continental 

 chemists ; and although chemical science has not largely 

 benefited by their researches, still society at largo lias 

 reaped great advantages in a sanitary point of view. 

 The general effect of the atmosphere on the animal 

 system, in reference to the processes of digestion, etc., 

 will be considered when we refer to the nature and 

 uses of food. 



We now proceed to speak of the combinations of 

 nitrogen with other substances. With oxygen it forms 

 the protoxide, binoxide ; hyponitrous, nitrous, and 

 nitric acids. The protoxide of nitrogen is, as its name 

 implies, a compound of one equivalent of nitrogen to 

 one of oxygen ; its equivalent == 2'J. It is a colourless 

 gas ; has a sweet taste, and is distinguished by its 

 peculiar effects in exciting laughter on being inhaled 

 into the lungs; hence its common name of "laughing 

 gas. " As many of our readers may feel inclined to try 

 experiments with it, we give full directions for its pre- 

 paration and inhalation. 



Experiment 43. Put two ounces of nitrate of ammo- 

 nia into a retort, or Florence flask with a bent tube. 

 Place this on the ring-stand, and let the stem of the 

 retort or flask dip into the pneumatic trough, or in a 

 basin. Instead of using cold water in these, a .solution 

 of two ounces of sulphate of iron or green vitriol, dis- 

 solved in a gallon of warm water, should be employed, 

 the object being to purify the gas as it passes through 

 the liquid. A gas jar, open at one end, and corked at 

 the other, is now to be filled with the water in the 

 trough, and arranged so that the stem of the retort 

 shall dip under its lower edge. A gentle heat is to be 

 applied to the retort, by means of the spirit-lamp, till the 

 salt contained in it melts. Laughing gas will then pass 

 over, and will gradually fill the glass jar of course 

 driving out the water. When no more gas passes over, 

 remove tho retort from the ring-stand, and let the jar of 

 gas stand over the water for some time, so that any 

 gases present, which, if breathed, might do serious 

 injury, may be absorbed. The gas may be collected in 

 a bladder, if one be screwed to the neck of the gas jar ; 

 but a simpler plan is as follows : Have ready a cork, 

 which will just fit the neck of the glass jar filled with 

 gas, and into this cork fit a piece of pewter tube, which 

 must pass through a hole bored in the cork. To this a 

 piece of india-rubber piping, about three feet long, should 

 be attached, ending in a small piece of metal tube, which 



