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On account of the ease with which the state of insensibility can be brought about, this gas is extensively employed as an anæsthetic, especially in dentistry.

Nitrous oxide is a gas which is moderately easily liquefied ; at o° C. a pressure of thirty atmospheres is required to effect its liquefaction.

Liquid nitrous oxide is colourless and mobile ; it boils at - 89.8°, and when dropped upon the skin produces painful blisters. When thrown upon water, a quantity of the water is at once converted into ice ; mercury poured into a tube containing a small quantity of the liquid is instantly frozen. An ignited fragment of charcoal thrown upon the liquid floats upon the surface, at the same time burning with brilliancy. If the liquid be mixed with carbon disulphide, and placed in vacuo, the temperature falls to – 140°. By strongly cooling the liquid, contained in a sealed tube, Faraday succeeded in solidifying it ; this may also be effected by the rapid evaporation of the liquid. The solid melts at – 102.7°, and if placed upon the hand causes a painful blister; in this respect it differs from solid carbon dioxide, which gasities without previous liquefaction.

Hyponitrous Acid, NHO.- This substance has not yet been isolated, being only known in its salts and in aqueous solution.

When a solution of potassium nitrate, or nitrite, is acted upon by sodium amalgam (an alloy of sodium and mercury), the salt is reduced by the nascent hydrogen, evolved by the action of the amalgam upon water, and ihe potassium salt of hyponitrous acid is left in solution

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The solution, which is alkaline, owing to the presence of sodium hydroxide, is then made neutral by the addition of acetic acid, and silver nitrate added. A yellow precipitate is thrown down, consisting of silver hyponitrite, Ag. VO.

When a solution of potassium hyponitrite is acidihed and then hented, the hyponitrous acid, which may be regarded as liberated by the acid, is broken up into nitrous oxide and water

2HNO=N,0+H,0. Nitrosyl Chloride, * NOCI. --This compound may be obtained by the direct combination of ritric oxide with chlorine

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* Tilden has shown that this is the only oxy-chloride of nitrogen that exists.

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Nitrosyl chloride is also readily prepared by the action of nitrosyl hydrogen sulphate upon dry sodium chloride, thus

(NO)HSO4+ NaCl=NOCI+ NaHSO4.

Properties.--Nitrosyl chloride is an orange-yellow gas, which easily con. denses when passed through a tube immersed in a freezing-mixture, to an orange-yellow liquid, which boils at about -8°. It is decomposed by water into nitrous acid and hydrochloric acid-

NOCI+H,0=HNO, + HCl. In a similar manner it is decomposed by metallic oxides and hydroxides, thus

NOCI+2KHO=KNO2+ KC1+H2O.

Nitrosyl chloride has no action upon gold and platinum, but it attacks mercury with the formation of mercurous chloride and the liberation of nitric oxide

2NOCI+ 2Hg=Hg,C1, +2NO.

CHAPTER VI

THE ATMOSPHERE

The atmosphere is the name applied to the gaseous mixte which envelops the earth, and which is commonly called the 1T. The older chemists used the word air much as in modern tims the word gas is employed ; thus they spoke of inflammable air. dephlogisticated air, alkaline air, and so on.

The air consists of a mixture of gases, the two chief ingredienis being nitrogen and oxygen. Lavoisier was the first to clear prove that oxygen was a constituent of the air, although Robert Boyle and others before him had shown that air was absorbed by metals in the process of forming a calx, and that the metal gained weight as the calx formed. When the fact that the air was cort posed of oxygen and nitrogen became established, various de vices were adopted to determine the proportion of oxygen in i Priestley's method was by means of nitric oxide. It depended upon the fact that when nitric oxide is mixed with air it combines with the oxygen, forming brown fumes which dissolve in the water. A contraction in volume therefore takes place, from which the volume of oxygen may be calculated. This method yielded results which seemed to show that there was considerable variation in the proportion of oxygen present in different samples of air, and the idea arose that the wholesomeness or goodness of the air #23 dependent upon the quantity of oxygen which it contained. Hence arose the term eudiometry, signifying to measure the goodness. Cavendish, on the other hand, as the result of a large number of experiments made by him, came to the conclusion that there was no difference in the samples of air that he experimented upon.

Since the time of Cavendish, eudiometric analysis has been brought to a state of great perfection and accuracy by Bunsen, Regnault, Frankland, and others. The conclusion to be drawn from the extended researches of these chemists is, that although the atmosphere certainly shows a remarkable uniformity of composition, there do exist perceptible, though very slight, variations

in the amount of oxygen present at different places and at different times. Samples of air collected from all parts of the globe, from

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mid ocean, from high mountain peak, American prairie, and crowded cities, show a variation in the proportion of oxygen ranging from 20.99 to 20.86. Angus Smith has shown that in foggy

weather the oxygen in the air in towns sometimes falls as low 3 20.82. Samples of air taken from crowded theatres have been found to contain as little as 20.28, while in many mines the amous averages as low as 20.26.

The mean proportions of oxygen and nitrogen in the atmosphere may be given as--

Oxygen . . . . . 20.96 parts by volume.
Nitrogen . . . . 79.04 ,

100.00 The composition of the atmosphere by weight was determined by Dumas and Boussingault (1841). In their method, air which was freed from carbon dioxide and moisture was slowly drawn throcy a glass tube containing a known weight of metallic copper, beatei to redness. The oxygen combined with the copper, forming coppe oxide, which was afterwards weighed, and the nitrogen passed into a vacuous flask, and was also weighed. The apparatus as employed by Dumas is seen in Fig. 53. B is a glass flask having a capacity of 10 to 15 litres, which was exhausted and then weighed. It was then attached, as shown, to the tube T, containing a knos weight of metallic copper, and which was also exhausted. The bulbs L contained a solution of potassium hydroxide, and the tubes f, solid potash, for the removal of atmospheric carbon dioxide. The bulbs O contained strong sulphuric acid, and the tubes t were filled with pumice moistened with the same acid, by means of which the moisture was withdrawn from the air. When the copper was heated and the cocks partially opened, air, free from carbon dioxide and moisture, was slowly drawn over the heated metal, which was thereby converted into the oxide. At the conclusion of the experiment the globe and the tube T were reweighed. The nitrogen remaining in tube T was then pumped out and the tube once more weighed. The difference between the two last weighings of the tube, added to the gain in weight suffered by the globe, gave the nitrogen ; while the difference between the original and final weights of the tube gave the increase of weight suffered by the copper, that is, the amount of oxygen. The result of numerous experiments gave the mean composition

Oxygen . . . . . 23 parts by weight,
Nitrogen · · · · · 77 ·

100

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