272 



THE POPULAE EDUCATOB. 



traces 

 traces 



When the aspirator is empty, the tubes are again weighed, and 

 thus from the increase of weight we learn the quantity of 

 moisture and carbonic acid gas which were in the volume of air 

 in the aspirator. 



The moisture in the air varies immensely. 

 The proportion of carbonic acid is generally about 55^ of the 

 whole volume of the atmosphere. 



Traces of ammonia and sulphuretted hydrogen are found near 

 towns especially ; and nitric acid is frequently present after a 

 thunder-storm. 



The average composition of the atmosphere is 



Oxygen 20'61 



Nitrogen 77 '95 



Carbonic acid '04 



Aqueous vapour . 

 Nitric acid 

 Ammonia 



Carburetted hydrogen 

 f Sulphuretted hydrogen 

 Intowns \ Sulphurous acid . 



The physical properties of the atmosphere not coming within 

 the actual domain of chemistry, we pass on to 



THE CHEMICAL COMPOUNDS OF NITROGEN. 



With oxygen, thia gaa produces five well-defined compounds : 

 Nitric anhydride. 

 Nitric peroxide. 

 Nitrous anhydride. 

 Nitric oxide. 

 Nitrous oxide, or Laughing Gas. 



The regularity of their formation offers the best of examples 

 :of the law of " multiple combination." It will be seen that the 



compounds are formed by the 

 addition of an atom of oxy- 

 gen, the quantity of nitrogen 

 remaining the same. Hence 

 the increase of the atomic 

 weights will be by the addition 

 of 16 ; and as atoms are indi- 

 visible, therefore there can be 

 no addition to the weight, save 

 in a multiple of 16. The law 

 of which this is an example, 

 is an evident deduction from 

 Dalton'a atomic theory, which 

 has been already explained. It 

 is known by the name of " the 

 law of chemical combination in 

 multiple proportion." Nitro- 

 gen only combines with oxygen 



N a O a 

 N a O 



jji- gj under peculiar circumstances. 



Whenever an electric spark 



passes through the atmosphere, their combination is effected. 

 Thus, after a flash of lightning, nitric acid is always found in 

 the air. We also find this compound is frequently formed 

 during the putrefaction of animal matter, and it occasionally 

 happens that some of the lower oxides of nitrogen are also pro- 

 -duced at the same time. 



NITRIC ACID, OR HYDRIC NITRATE (HNO 3 ). 



As all the other compounds of oxygen and nitrogen are 

 derived from this acid, we shall speak of it first. 



In the symbol given above for nitric acid, N 2 O 5 , it will be 

 een that there is no hydrogen ; and therefore N 2 O 5 is, strictly 

 speaking, not an acid ; it only becomes one when joined with 

 water. Thus the symbol will be H S ON 2 O 5 = H 2 N a O e . This 

 is usually written HNO 3 . 



T0 prepare nitric acid, it is necessary to act on one of the 

 plentiful nitrates, potassium or sodium nitrate (KNO 3 , or 

 NaN0 3 ), with sulphuric acid. In the laboratory it is usual to 

 place in a retort equal weights of potassium nitrate (saltpetre) 

 and sulphuric acid, and then distilling in the ordinary way with 

 Liebig's condenser, the action is 



2KNO, + H a SO t = KjSO^ + 2HNO 3 . 



For commercial purposes sodium nitrate is used, being cheaper 

 and yielding 9 per cent, more of the acid ; but as a greater heat 

 is required, the nitric acid, which distils over, always contains 

 sulphuric acid. 



Fig. 32. 



The liquid thus obtained fumes in the air, and ought to be 

 colourless, but is usually tinted yellow by the presence of the 

 next lower oxide the yellow gas nitrous acid. 



It easily parts with some of its oxygen, and is therefore a 

 good " oxidising agent." If, for instance, a piece of copper be 

 thrown into a little nitric acid, violent action shortly commences, 

 especially if the liquid be warmed ; dense red 

 fumes are given off, and the copper becomes 

 cupric nitrate, blue vitriol (Cu2NO 3 ) ; by 

 diluting the solution and slowly evaporating, : 

 the blue crystals of the salt maybe obtained. 

 Its readiness to part with its oxygen ia 

 also shown by dropping the acid into hot, 

 finely-divided charcoal, which begins to 

 burn vividly. If the nitric acid be required 

 in a very powerful form, some sulphuric 

 acid is added, which deprives the nitric 

 acid of its water. Oil of turpentine bursta 

 into flame when such a mixture is poured 

 into it. All the metals, except gold and 

 platinum, are attacked by this acid. Thus 

 we have a means of deciding between real gold and a mere elec- 

 trolytic gilding. On the former a drop of the acid remains un- 

 changed, whilst in the latter case it becomes green. 



To test far the presence of Nitric Acid. If any body be sup- 

 posed to contain thia acid, whether in a free or in a combined 

 state, get the substance into solution, add an equal quantity of 

 sulphuric acid, which will set the nitric acid free, raid carefully 

 add a concentrated solution of iron sulphate (green vitriol). If 

 any nitric acid be present, a black ring will be formed at the 

 place where the two liquids meet. 



The anhydrous acid (N 2 O 5 ) may be obtained by a method pro- 

 posed by Natterer. Dry chlorine gas is passed over crystals of 

 silver nitrate, the action being 



.2AgNO, + 2C1 = N a O s + O + 2AgCl. 



The silver chloride remains in the bulb, the oxygen escapes, and 

 the anhydrous nitric oxide condenses in a U tube, which is 

 placed in ice, and which ia joined to the bulb containing the 

 silver salt. The substance collects as a white crystalline solid. 

 It must be kept in a sealed tube, or it abstracts water from the 

 air, and becomes liquid nitric acid. Hence it is only a chemical 

 curiosity, and of no practical use. 



In concluding this lesson we add tests for ascertaining the 

 purity of commercial nitric acid, for every care ought to be 

 taken that laboratory chemicals are pure. Since the process for 

 obtaining nitric acid in large quantities is carried on in a cast- 



Fig. 33. 



iron retort, and at a high temperature, the acid generally con* 

 tains sulphuric and hydrochloric acids. 



To discover the presence of the Srst substance, take some 

 of the suspected acid, dilute it with four times its volume 

 of water, and add a little barium chloride. The insoluble 

 barium sulphate will be formed if the slightest trace of sul- 

 phuric acid be present, and it will exhibit itself in a white pre- 

 cipitate. 



In testing for hydrochloric acid, it is not necessary to dilute 

 the nitric acid, but add a few drops of a solution of silver nitrate, 

 and ff any hydrochloric acid be present, then there will be a 

 white curdy precipitate of silver chloride. The two reactions 

 are thus expressed : 



H a SO t + Bad., = BaSO 4 + 2HC1, 

 and HC1 + AgNO, = AgCl + HNO,. 



