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removed, the residue consisting of krypton and xenon was easily solidified. Under these circumstances it was found that the krypton could be withdrawn by pumping, for at the temperature of liquid air solid krypton is appreciably volatile, while the solidified xenon is practically non-volatile. In the estimation of the discoverer, air contains only about one part of krypton in one million parts ; while of xenon the proportion is about one part in twenty millions. The boiling and melting points of krypton are – 151.7o and 169 respectively, while those of xenon were found to be – 109.1°and – 140°.

The light emitted by krypton, under the influence of the induction spark, is a yellowish-green colour, while that given by xenon under the same circumstances is more of a sky-blue.

The most prominent lines in the spectrum of krypton are two very near together in the red, one bright yellow line and one strong green line ; besides which there are a few in the blue and violet. The brilliant green line (wave length, 5570.5) has attracted special notice, as it is considered highly probable that this line may prove to be coincident with the chief line in the aurora spectrum.

The spectrum of xenon, like that of argon, is markedly different as the electric discharge is modified. With the intermittent discharge the prominent lines are four in the red end, and a number of strong lines in the blue and greenish-blue. With the "jar" discharge the red and biue lines become very reduced or altogether disappear, and their place is taken by a number of lines in the bright green.

The relative proportion in which these gases are believed to be present in the atmosphere has been estimated provisionally by the discoverer as follows :

Helium, i to 2 parts per 1,000,000 of air.
Neon, , , , 100,000 ,
Argon, 0.937 part 1

1 00
Krypton, i „

1,000,000 , Xenon, 1

» 20,000,000 , Or, expressed in parts per 1000, to compare more readily with the figures in the table on page 256 :

Argon . . . . . 9.37
Neon . . . . : 0.01
Helium

0.001
Krypton

0.001 Xenon. , , , 0.00005

The following table (Ramsay) gives the latest physical constants for the members of this strange group of new elements.

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The members of this group, while exhibiting many close resemblances, such as the monatomic nature of their molecules, their remarkable inertness, &c., show also that gradation of properties which is met with in other natural groups of elements. This appears by the results tabulated above, as well as by such other properties as the refractive indices, atomic volumes, &c.

CHAPTER VII
COMPOUNDS OF NITROGEN AND HYDROGEN

THREE compounds of nitrogen with hydrogen have been prepared, namely :

Ammonia . . . . . NHA-
Hydrazine . . . . . N,H, or (NH.).
Hydrazoic acid . . . . N H or HN3.

AMMONIA. Formula, NH3. Molecular weight=17.04. Density=8.52. History.-Salts of ammonia, and also the aqueous solution, were known to the alchemists. It was termed by Glauber, spiritus zolatilis salis armoniaci, being obtained by the action of an alkali upon sal-armoniacum. Subsequently, when ammonia was obtained by the destructive distillation of such refuse as hoofs and horns of animals, the name spirits of hartshorn was applied to it. The actual discovery of gaseous ammonia was made by Priestley (1774), when he collected the gas, evolved by the action of lime upon sal-ammoniac, by means of his mercurial pneumatic trough Priestley named the gas alkaline air.

Occurrence.-In combination as carbonate of ammonia it is present in small quantities in the air, derived by the decay of nitrogenous animal and vegetable matter. As nitrate and nitrite it is found in rain-water. It is evolved, along with boric acid, from the fumaroles of Tuscany (see Boric Acid), and is found as chloride and sulphate in the vicinity of active volcanoes.

Modes of Formation.-(1.) Ammonia can be synthetically produced by submitting a mixture of nitrogen and hydrogen to the influence of the silent electric discharge (Donkin). The amoun: of ammonia so obtained, however, is extremely small, and can bes: be shown by passing the gases, as they issue from the “ ozone tube," through a cylinder containing a small quantity of Nessler's

solution.* in a short time the solution will begin to show a yellowish-brown colour, indicating the presence of traces of ammonia.

(2.) Ammonia may be prepared by gently heating any of its salts, with either of the caustic alkalies, potash or soda, or with slaked lime. The salt most commonly employed is the chloride. When this is mixed with an excess of slaked lime, and the mixture gently heated in a flask, ammonia is evolved, and calcium chloride and water are formed

2NH4C1+CaH,02=CaCl,+2H,0+2NH3. The gas may be dried by being passed through a cylinder containing lumps of quicklime,t and may then be collected either by upward displacement or in the mercurial trough. On account of its extreme solubility it cannot be collected over water.

(3.) Ammonia is formed by the action of nascent hydrogen upon salts of nitrous and nitric acid, thus

NaNO3+ 4H,=NaH0+24,0+NHz. This method is often made use of in the quantitative estimation of nitrates in drinking water.

(4.) When nitrogenous organic matter is subjected to destructive distillation, that is, strongly heated out of contact with air, ammonia is formed ; hence when coal, which usually contains about 2 per cent. of nitrogen, is distilled in the process of the manufacture of ordinary illuminating gas, one of the products of the decomposition is ammonia. The "ammoniacal liquor" of the gas works is the source of all ammonia salts at the present day. The liquor is boiled with milk of lime, and the ammonia thus expelled is absorbed by sulphuric acid. The ammonium sulphate so obtained is purified by recrystallisation.

Properties.-Ammonia is a colourless gas, having a powerfully pungent smell, and a strong caustic taste. It is lighter than air, its density being 0.589 (air=1). Ammonia possesses the property of alkalinity in a very high degree ; it turns red litmus blue, and yellow turmeric brown. The gas is unable to support combustion,

* A solution of mercuric iodide in potassium iodide, rendered alkaline with potassium hydroxide,

+ The usual desiccating agents, namely, sulphuric acid, or phosphorus pentoxide, are inadmissible in the case of ammonia, as this gas at once unites with such compounds.

and is irrespirable. Under ordinary conditions ammonia is not combustible, but if the air be heated or if the amount of oxygen be increased the gas will then burn with a flame of a characteristic yellow-ochre colour. This behaviour of ammonia as regards com

bustibility is most conveniently
illustrated by means of the ap-
paratus shown in Fig. 56. A
stream of the gas obtained by
gently heating a quantity of the
strong aqueous solution in a
small flask is delivered through
a tube which is surrounded by
a wider glass tube. Through
the cork which carries this tube
a second tube passes, through
which a supply of oxygen can
be passed. On applying a
lighted taper to the jet of am-
monia as it issues from the tube
it will be noticed that the gas
burns in the heated air round
the flame of the taper, but is
unable to continue burning when

the taper is withdrawn. If now FIG. 56.

a gentle stream of oxygen be

admitted into the annular space between the two tubes the ammonia readily ignites, and continues to burn with its characteristic flame. On cutting off the supply of oxygen the flame of the burning ammonia languishes and dies out.

Ammonia is extremely soluble in water ; I c.c. of water at o° C. and at the standard pressure, dissolves 1148 c.c. of ammoria. measured at o° C. and 760 mm. The solubility rapidly decreases as the temperature rises, as will be seen by the following table :-I c.c. of Water at

Grammes, NH,. cc. at o° C. and
760 mm. Dissolves

760 mm.
At oo. · · 0.875 . . . 1148
,, 8° . . . 0.713 . . . 923
, 16° . . . 0.582 . . . 764
, 30° . . . 0.403 . . . 529
, 50° . . . 0.229 . . . 300

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