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Otago Witness, Issue 1489, 29 May 1880, Page 21

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PEOPESSOB, ELACK'S SATURDAY LECTU&ES.

The third lecturs ot this course was delivered on the 22 >;d inst. Tho subject was oxygen (continued from last lecture), its allotropic modification, ozone, and nitrogen.

The oxidising power of atmospheric oxygea on the metals in a state of very fine division was shown by projecting some veiy fine powder of metallic leai into the air. The particles of lead ignited, and burned with a bright red flash. The powder wa-) prepared by heatiag ia a te3t-tube some tartrate of lead at a temperature a little below the fusing point of tho metal. This liberates the lead in the state of very minute particles mixed up with charcoal derived from the tartaric acid of the salt.

In the same way it wai pointed out that metallic iron, if reduced from the oxide at a high temperature by a current of hydrogen, would also ignite when thrown into the air. The reducing action of the hydrogen ia thna Bhown by »n equation :—: —

Fe.A+6H=3H 2 O+2Fe iron ru3t + hydrogen = steam + metallic iron; A substance that tb.ua ignites spontaneously ia the air is called a pyrophorus, from two Greek words meauitig fire-carrier. The oxidising' power of nascent oxygen was also shown by letting a drop of strong sulphuric acid (oil of vitriol) fall on a mixture of sugar and chlorate of potash. At the first oontact a violent combustion began, accompanied by the evolution of clouds of condensing steam and a violat coloured flame ; the violet colour, aswas explained, being due to the combustion of particles of the potasaium of the chlorate. At the close of the experiment both the sugar and the chlorate had disappeared, and in their place there was nothing seen but a swelled mass of charcoal derived from the sugar, The lecturer then put some small bits of phosphorus into r tall glass full of water. He then covered the phosphorus with chlorate of potash. A few drops of strong sulphuric acid were then led by a long-stalked funnel down through the water, and delivered just over the chlorate. Immediately a hissing, crackling noteo wai heard, and the phosphorus was seen to be molting and burning brilliantly under the water. In both the experiments the oxygen was derived from the chlorate of potash, the liberating agent being sulphuric acid. A solution of protosulphate of iron (copperas) was then put into a violet red solution of permanganate of potash. The iron was instantly oxidised at the expense of the permanganate, the latter becoming colourless from the loss of some of its oxygen.

The presence of free or uncombined oxygea in the air was proved by its blackening effect on a mixture of chloride of manganese and caustic potash ; also,_ by its blackening a solution of pyrogallic acid in caustic potash j and at a f nrthfir stage by converting the colourless nitric oxide gas, NO, into the ruddy brown gas, peroxide of nitrogen, NO 3 . Ozone wai then taken up, Thiß was explained to be oxygen in a Btate of great concentration and excitement. It was stated that in the form of czane 3 atom 3of oxygen occupy the space occupied by 2 atoms of ordinary oxygen. It is therefore 50 per cent, heavier than oxygen. It is, however, only an allotropic form of oxygen, possessing all the properties of that gas in a state of greater activity^ It is a more powerful oxidiser than ordinary oxygen, causisgthe metalH to runt more rapidly, rußtiug even silver and otner metals which re« main untarnished ia air or oxygen. Unlike oxygen, ozone ip said to be quite insoluble iv water.

It is obtained by passing a nerie3 of eleotrio sparks through air or oxygen. In this way, no doubt, roost of the oa ma of the atmosphere is produced by thunderstorms : henco the greater purity of tho air after thunder, as the cz >ne by its superior energy will bura up the organic gaseous and suspended impurities. Ozone is aIHO produced by the electrolysis of water (tho splitting up of water by a current of electricity), part of the oxygen produced in thiß caseshowing itself ia the form of ozone. The lecturer made someczone by two methods, first by suspending a piece of phosphorus in a moist bottle for about half an hour, and again by plunging a very hot glass rod into the vapour of ether in a large glass jar. Tbe Blow combnation of the phosphorus and the other respectively was said to bo the producing cause in these ca^es, The test by which the czone was identified was the introduction into the jar and bottle of a piece of blotting-paper previously wetted with a solution of potassic iodide (iodide of potajh, XI) and au emulsion of starch.

The ozone liberated iodiue from the iodide thus :-O 3 +2KI K 2 O + 2l+O 2 . This free iodine then coming in contact with the starch made it blue, and the blue colour waa the test. It was explained, however, that chlorine gaa and nitrous acid would have the same effect. Tho ivb vi.cc of the3e haa therefore to be established before this test ia applied for ozone. Another test for ozone, to which tho same objection does not apply, is red litmua, moistened with a solution of potassic iodide. Ozone will show its presence by forming potash, K 2 O, as shown by lust equation, and the potash will make the red paper blue. AmmoDia, however, will have the same effect as > zone in this case, and its absence has to be proved by a second red litmus paper, without the iodide, remaining red before the presence of ozone can be affirmed. Thos-3 ihree tests, taken together, will establish the presence or absence of this gax The lecturer proceeded npxi; to the consideration of nitrogen and it* compounds. This gas is the mort abundant constituent of atmospheric air, constituting 77 per cent, of it by weight, or 79 per cent, by volume, the remainder being almost entirely oxygen, with a little cwhonic acid and the vapour oE water. . In tho air these ga3as aro mixed together, not combined. The most important purpose that the nitrogen serves in the atmosphere ia doubtlesn the dilution of the oxygen, which other wine woula lie altogether too energetic in its oxidising action. It serves the purpose also of givintr body or bulk to the air, thus rendering the circulation, more porfecfc, and moderating and regulating the temperature, and sorving, no doubt, many imi.ortant purposes besidas. Niirogeu silsi nccurß combined in the flo3h of animaK in fill plants and in some minerals — a3 saltpetre, coal, &o. NHropen r, i.ho L'^st active of the elementary gases It ha-s no yrouerties of its own to speak of. It i'-i characterised chiefly by the want of prcporties. It doe* not support combustion nor sustain animal life ; it is neither alkalino nor* acid, therefore dues not affect tho colour of litmus. It is virtually insoluble in water, owe gallon of it r^quiiinsr GO gallons of water to hold it in unlution. Unlike carbonic acid, ifc' doeti not precipitate lirae m ator. It is not combustible. It is not even poisououp. A burning substance introduced into ifc ia instantly extinguished, not on account of the presence of nitrogen, but owing to the absence of oxygen. For the same reason, a living animal placed fy the pure gas dies of suffocation,

But although thus inactive and indifferent m the free state, many compounds of which it forms a part have most extraordinary properties, and constitute some of the most ene.g.-lic, poisonous, and explosive substances knowu. For example, ammonia gas (NH 3 ) is a powerful volatile alkali, quite irrespirable, and possessing an intolerable pungent odour and caustic taste, neutralising most effectually the strongest acids. So nitric acid (HNOj) is one of the most corrosive acids known, attacking most metals and all organic matters with the utmost energy. In prussic acid (HON) we have another well-known compound, of which nitrogen forma an essential part, one of the most deadly and most rapid in its action of all poiaons. We have it again in conjunction with oxygen, forming the most essential constituent of such explosives as niiro-glycenne and gun-cotton; the former known under various names when diluted with sand, broken glass, &c, &c, as dynamite, Utiwfracteur, &c. We have it also in the alkaloids, in all of which it ponstitutesthe essential element. Of these it will be sufficient to mention strychnine, brucine, nicoiin, narcolin, morphia, quinine— all jvery energetic in their pbyaiological action. And lastly, though itself a colourless gaß, we have it as an essential element in the whole series of the now wellknown varied and brilliant coaUtar colours. When required for laboratory experiments, it is best prepared by removing the oxygen from ordinary air. This may be done by confining a living animal— say a cat or rabbit— in an isolated portion of air. The aDimal breathes as long as it can, at every inhalation removing come of the oxygen which it gradually displaces by carbonic acid, exhaling the nitrogen, and leaving it just as it found it. After the death of the animal, the vitiated air will be carbonic acid and nitrogen : or the oxygen can be removed by the combustion of charcoal or sulphur, or, better still, phosphorus in a confined portion of it. m In each case the oxygen combines with the combustible, forming respectively carbonic add, sulphurous acid, and phosphoric acid. The equations are : o+Nx+0 s =o0 a +Nx Carbon+air = Carbonic acid+Nitrogen S+Nx+O 5 =SO 4 +Nx Sulohur+air= Sulphurous acid+Nitrogen P 2 +Ny+O 5 =P 2 O s+Ny5 +Ny Phosphorus+air-phoaphoricacid+nitrogen. These three so-called acids should, however, be termed anhydrides, carbonic anhydride, sulphurous anhydride, and phosphoric anhydride, their chief property being their readiness to combine with water to form the acids of the same name. By each of these four methods of getting nitrogen gas the mixlure obtained is Bhaken np with water, which rapidly dissolves the OO 5 , SO 2 , and P2P 2 O 5 , leaving the saitrogen nearly pure. The professor here confined Rome air in a large jar over water, and ignited a n'e?e < f dry phosphorus in it. The oombuHii.m piocewded with the formation of de- <c whito clouds of P2P 2 O 5 , which very soon after the burning censed were dissolved in the water. _ At first the water was depressed in theiuuer jar. This was explained by the expansion of the air at the high temperature produced by the burning phosphorus. At the end of the experiment, when the residual gas had cooled down and the white fumes had dissolved, the water was seen to be standing ata higher level in the inner jar. TWb was explained to be due to the pressure of the air on the surface of the water in the outer vessel, thus forcing the water up into the inner jar to occupy the place of the oxygen, which hart been removed to form J? 3 O 5 . The water thus occupied about a fifth of the space previously occupied hy the air, thus showing that one fifth of the air (aatt^ly, all the oxygen) had diuappeared. A timing splinter of wood was then lowered into >ha residual nitrogen, when it was instantly ixtinguished. The degree of solubility of oxygen and nitrogen in water proves that they are merply mixed in atmoßphtric air in this way:— 2s gallons of water at ordinaiy temperatuies dissolves one gallon of oxygen and less thaw half a gallon of nitro:>«n, and thi3 is the proportion in which we find these gases in water exposed to the air. But ia air there are four gallons . f nitrogen to eveiy one tral'on of otygen.^ Also, water, when it dissolves c> mpouuds, dissolves them as a wivta— -that is, dissolves them as compounds, every element having in solution the same proporiioo to every other that it has in the undissolved compound. If, therefore, air were a compound, and if water dissolved it, we would find in water four volume* of nitrogen for one volume of oxygen, instead of half a volume of nitrogen to one volume of oxygen. This is, however, only one of many proofs of the mixed nature of atmospheric air. Nitrogen forms five compounds with oxygen, their names and symbols being as under :— N 2 O, nitrous oxide, or nitrogen monoxide, or laughing gas. N2N 2 O S , nitric oxide, or nitrogen dioxide. N2N 2 O 3 , nitrous anhydride, or nitrogen trioxide. N2N 2 O 4 , nitrogen peroxide, or nitrogen tetroxide. N3N 3 O 5 , nitric anhydride, ,or nitrogen pentoxide.

The second and fourth members of this aeries are Bometimes written NO and NO a . The odd numbers of these, namely, the Ist, 3rd, and stb, are anhydrides, and therefore combine with water to form acids, thus :— NqO+HjO=2HNO= hyponitrous acid. NoO,+H 2 0=28N0 2=2 = nitrous acid. ;NA+ H 2°=2 HNO 3= nitric acid.

Of these acids, however, hyponitrous acid is still unknown. Laughing gas or nitrous oxide CNSO) was made by heating nitrate of ammonium (NH,N0 3 ) in a glass retort. The Bait first melted and then broke up into water and nitrous acid, as by the equation :— NH-,NO 3 heated =N 2 O+2H 2 O Nitrate 01 ammonium = nitrous oxide + water. The steam and nitrous oxide were conducted through warm water in an inverted jar, in which the steam was condensed and the laughing gas collected. The properties of this gas were explained, and its übo in surgical operations and in dentistry. A blazing splinter of wood was extinguished, and with the tip still red, was introduced into the gas, when it was instantly rekindled, and burned with little less brilliance than it had burned in pure oxygen. The heat of the red-hot chip had decomposed the gas into a mixture of two volumes of nitrogen and one volume of oxygen, thus :—

NsO=N s +O, which is seen to be oxygen hti = nr+n

Jess diluted than in air : hence the energetic combustion.

The second of these gases, nitric oxide (NO) was made by the action of nitric acid on metallic copper in a glass retort, thus :—: —

SCu+SHNO^Ou (NOj) +4ET 2 o+2 a NO

The gas itself is pejfectly colourless, but on mixing with the free oxygen of tho air in the retort, at the moment of production it assumed another atom of oxygen, becomirg NOo — a brown gas very soluble in water. In passing through the water to the receiver this brown gas was dissolved, ard tho colourleHs gas (NO) nitric oxide, being insoluble in water, was collected in two inverted jars, in tbe usual way. Into one of thepe jarp, still standing inverted over water, some oxygen was admitted. Imjnedi&tely the ruddy gas NO 2 was formed and f ftjpidly owsolyed jin the water, which then was

forced tip by atmospheric pressure to occupy its place. The water was then tested with bluo litmus paper, when, it immediately turned rod, thiw proving the presence of an acid. Into the other cylinder were introduced a few drops of Usulph ide of carbon. The cylinder was shaken up and a )igh>, applied, when along greeniah-blue flame Suot out of its mouth. The flame was due to the burning of the sulphur of the bisulphide, thug : —

CS 2 +GNO = CO 2 +2SOo+6N Bisulphide 01 carb»u + nitrogen ditroxide = carbonic acid + sulphurous acid + nir,rogen. The students weca then divided into two sectious, aa there was not room for them, to experiment iv the lecture lo^m. Guided by the Professor and Mr Catian, they applied the usual tests to solutions containing ihe metals gold, cadmium, copper, and arsenic To each solution was first a°ided a little hydrochloric acid (RCI). This did rot apparently affect the solutions in any way. To the same test-tubes was then added some hydrosulphurio acid (*ulplmre.,ted hydrogen, HSS).H S S). Tbi3 gave a precipifcar,o ia each case. With gold and copper tho precipitates were black; with cadmium and atseuic tuey were yellow. The black of the gold and the yellow of the arsenic were found to be soluble in sulphide oi ammonium (NH,HS); whilst those of copper and cadmium were insoluble in that liquid. The (/old wus finally and conclusively identified by tho addition of some solution of protosulphate of iron (the copperas of druggists, FeSd), with which the gold showed, by transmitted light, the green-blue colour seen in looking through gold-leaf; and, by reflected light, a yellowish-brown colour. The copper was identified first by giving with two drops of ammonia (NIL,) a blue precipitate, which on the addition of more ammonia cleaved up to a beautiful deep blue transparent liquid. The copper was further identified l»y the copper colour assumed by a knite-blei.de or a needle on being placed in vhe solution, acidified by a little hydrochloric acid. The cadmium was identified by giviLg a white with a little ammonia, which dissolved in excess of that liquid. The presence of arsenic in the fourth glahs was proved by giving a, gr^en precipitate on being added to a solution of sulphate of copper to which two drops of ammonia had bo«-u added. This green is known as Scheele's green, and is the colouring matter of some w.UI-pai>er<i.

The subject of next lecture was announced to be nitric acid and ammonia, and, if time will admit, an introduction to carbon The metals to be tested are antimony, tin, and mercury in its higher state of combination.