Lectures.

Otago Witness, Issue 1488, 22 May 1880, Page 22

Lectures.

PROFESSOR BLACK'S LECTURES ON CHEMISTRY TO TEACHERS.

Professor Black commenced hia second course of lectures on " Chemistry " to teachers on Saturday, the Bth instant. There was a large atteudance, the lecture-room heing crowded. Up-country districts were well represented, notably Lawrence, Balclutha, Milton, Palmerßton, Waikouaiti, and around Oamaru. Nearly one-half the class were ladiea. . The Professor, in opening the course, after expressing the high satisfaction he felt in seeing so large an attendance of teachers, explained the immediate object of the lecture^ to be preparation in chemistry for the University matriculation examination, and for certificates in class D ; but besides, he had a wider and a higher object still, namely, to advance and popularise the study of . chemistry. After defining the province of chemistry, and pointing out its application to the arts and industries of everyday life, he stated that there are known to be about 65 elements, or simple substances, incapable of being broken up, by any means hitherto tried, into anything different from themselves. As examples of elements we have gold, which by any known means cannot be divided or split up, or separated into any other kind of matter than gold ; so with the other metals. So with oxygen, hydrogen, and the other non-metallic elements. Every substance or kind of matter known to us is built up of these 65 elements, Table salt, for example, U composed of the metal sodium and the gas chlorine united together; so water is composed of the gases hydrogen and oxygen ; so limestone is composed of the metal calcium and the non-metftllic!elemenls carbon and oxygen. The present course of lectures deals with only 14 of the elements, namely, those termed nonxnetallic. They are :—

I have nrranged those elements iv this order for good rensoua. Hydrogen in placed nt the head of the list, because it should not be there

this order :e f l nt the t be there

[at all. Although it is usually treated as a non- ! metallic element, it behaves like a metal. It is really a gaseous, light metal, but as a knowledge of its properties is essential to th<* successful study of thero - metallic elements, we shall take it up at the beginning of the course. The four elements next on the list I hava bracketed together into a group, binding them into a family because there is a strong family likeness between them. In the same way and for the same reason I have bracketed the next four together, constituting the incs, or the family of the halogens, or salt-makers, because they form simple salts by direct union with the metals— aB NaOl table salt, NaF fluoride of scdium, Naßr bromide of sodium, Nal iodide of sodium. The third group bracketed constitutes the family of the ons— carbon, boron, silicon— also showing many points of resemblance to each other. Lastly we have nitrogen and phosphorus bracketed. These are two members of a family of five ; as the other three, however, belong to the metals we shall not introduce them at present. The number of dashes f ) affixed to the symbols in this table indicates the atomicity of the elements. By the term atomicity is meant their relative value in forming compounds. For example, Cl* has one atomicity, indicated by one dash, CP can therefore combine with one atom of IT forming the compound HOI = hydrochloric acid. 0" has two atomicities, indicated by two dashes, 0" can therefore combine with two atoms of IT forming HHO or H2OH 2 0 or H— o— H= water. So in the same way N w has three dashes, and can take three atoms of IT forming H S N or NHHH or NH^ = ammonia. In the same way carbon Ov"O v " with its four atomicities can take four atoms of H, and form OHHHH or 0H 3 = marsh gas or firedamp. It must be understood that the atoms of the elements taken in the gaseous state and measured at the same temperature and pressure, ara all (with four exceptions ho far as known) of the same Bize, The four exceptions are : Phosphorus and arsenic, mercury and cadmium — the atoms of the first two mentioned occupying only half a volume, and of the lasb two two volumes ; whereas the atoms of all the other elements, so far as known, occupy one volume. By experiment it has been found that oxygen gas is 16 times as heavy as hydrogen gaa at the same temperature and pressure. But the atoms of these two gases are of the same size— that is, occupy the same volume, therefore an atom of oxygen is 16 times as heavy as an atom of hydrogen. In the same way an atom of chlorine is shown to be 35^ times as heavy as an atom of hydrogen, and an atom of nitrogen 14 times as heavy as an atom of hydrogen. For convenience, then, assume the weight of an atom of hydrogen as oue unit (you need not define the unit), and it follows that the weight of an atom of oxygen is 16 of these units j an atom of chlorine, 35 J of theae units; and of nitrogen, 14 of these unite We say, therefore, that the atomic weight of H is = 1, of 0 =* 16, of N = 14, of 01 = 35& ; and so on for the other elements— the symbols H, 0, N, 01, &0., &c, representing one atom of each element. When we desire to express 2 atoms of an element, say of 0, we write the figure 2 below to the right of it, thus— 0 2 . In this way 03,0 3 , 04,0 4 , 050 5 will mean 3, 4, 5 atoms of oxygen respectively. Rover ting to the atomicities for a moment, a dash may represent an arm or hand, by which an atom of an element may seiz9 the arm or hand of another element, thus— (HMCI)=HCI, @-@-@ = HSOH 5 0 (H)-(C) X=GH (°) @) (0)=GO & So much for the elements as a whole. Pass on now to consider them in detail, beginning with hydrogen. The name hydrogen means the water-for-mer, because it is one of the constituents of water, and when burnt in air or oxygen the product of the combustion is water. Hydrogen constitutes one-ninth' part of water by waighr. It exists in every acid ; it is also an abundant constituent of all plants and animals, and of the gaseous liquid and solid hydro carbons, as well as of many other substances. Hydrogen has no colour, taste, nor smell. It in the lightest substance known, atmospheric air being 14 4 times as heavy. It is almo3t insoluble in water. It was liquefied and solidified nearly two years ago by the application of intense cold and very great pressure. When mixed with half its bulk of oxygen the mixture explodes violently on the introduction of a naked light or the passage of an electric spark. The explosion is caused by the rapid burning of the hydrogen to form water, H2O.H 2 0. Many metals have no effeot on water at any temperature. Several metals, however, as iron, zinc, nickel, cobalt, &c, decompose steam (water in a gaseous state) at high temperature, the metal combining with the oxygen and liberating the hydrogen. Other metak— for example, potassium and sodium — decompose water in the cold state, combining with its oxygen and liberating the hydrogen. The Professor here cut a piece of the metal potassium (a soft, light, leaden-coloured metal) and threw it into a basin of cold water. The metal floated on the water, being lighter than water ; it rushed about continuously on the surface, a hissing sound was heard, due to the liberation of hydrogen ; the liberated hydrogen j burst into flame, which was coloured pinkviolet, due to the combustion of part of the potassium itself. Tho water, after the experiment, was found to have the power of changing the colour of red litmus paper blue, indicating the presence of an alkali, or base. The base in this case was caustic potash. The following equation shows tho reaction :—: — H2O+K=KHO+HH 2 0+K=KH0+H Water +potassium=caustic potash+hydrogen. The same experiment was repeated with the metal sodium Na in hot water, and the saino phenomena were obnnrvod, except that in this case the flame was yellow. A piece of sodium was then wrapped in blot-ting-paper and introduced under the mouth of an inverted beaker-glass full of water. Hydrogen %va<) then liberated &% before, but instead of being burned, it was collected in the inverted glaBS, displacing the water as it rose. It was afterwards exploded by admitting sonio air and applying a naked light. Hydrogen is best made on the large peal"} by the action of dilute sulphuric acid on zinc, as shown by the equation — Zn+H 2 SO4=ZnSO<+H 2 Zinc + sulphuric acid = sulphate rf zinc x hydrogen ; or of tulphuric acid ia irnn thus — Fe + H2SO,H 2 SO, == FeSO 4 + H2H 2 Iron + sulphuric acid = nulphate of iron x hydrogen ; or of hydrochloric acid (muriatic acid) on zinc or iron thus — Z + 2HCI = ZCJ a + 2TT Eh + 230] = I<Vr]. 2 + 2 'l forming chloride of z"nc and ch!or<<!n of iron respectively. When required pf-rfpet-.lv rive ;'. j< ' eat made by decomposing ocicJuUtod wutee by a

current of electricity. This experiment was in progress during the whole of the lecture, there being collected about four cubic inches of hydrogen and two cubic inches of oxygen, these being the proportions by volume of the two gases in water. The hydrogen so collected was tested by exploding it with air, and the oxygen by the brilliant com. bustion it maintained with charcoal. The lecturer then explained how hydrogen gas can be made on the large scale by passing a current of steam (H 2 0) through a gun-baml loosely filled with scraps of filings or shavings of iron and maintained at a red heafc in a suitable furnace ; the equation being— T. , ff 3 +,H 3 O=F 3 0,+ BHB H iron + steam = Magnetic oxide of iron + m , , , hydrogen, ±he oryhydrogen blowpipe was exhibited, and it was stated that the heat evolved by the combustion of one pound of hydrogen is sufficient to raise the temperature 34,0001b of water one degree Centigrade, or sufficient to heat 34 gallons of ice-cold water to boiling-point. A jet of hydrogen was burned and a cold gUvs cylinder held over it, when minute drops of water were seen condensed on the surface of the cylinder m a very short time. exhibltS Und te fla ? e^ all W en tube waß ak° exhibited. The note differed in pitch on every change of tube, but in each case it was a musN cal note,, and due, as the Professor said, to a oXintervals 0 P losions occ « r ™g * synchro* The next subject taken up was oxygen This is the most abundant and probably the moat Widely-diffused element known of the earth, 88 8 par cent, of water, and 23 per cent, of atmospheric air. The element was discovered by Priestley in England m 1774, and independently about the same time by Scheele, of Sweden. It is however, to Lavoisier, a distinguished French chemist during the last 30 years of the last century, that is due the investigation of the important part played by oxygen in combust tion and allied processes. He demonstrated, by the help of the balance, that combustion is a process of oxidation, that when a substance bums it mere y combines with oxygen, the heat and the light produced being merely resuits or effects of the acid union. The name oxygen means acid-maker -not by any means an appropriate name, as many acids ttt C adSS ygen ' aQd TWy »"* ° Xidea Thw gas was firat made by Priestley by heating red oxide of morcury. At a dull red heat the oxygen and the mercury are separated. t n l,l/° latilißed> ? the y are P^ed into water the mercury condenses, and the oxygen being momdenaible, may be collected in the usual way. ° The equation is— HgO heated = Hc+o. ' that h o e f:x°yge o nlt ght ° £ ""^ " 2 °°' * nd 216 pounds of the red oxide HgO will there, ou/yg'en 2 °° P ° UndS ° f """WvidM pSSS Black oxide of manganese, or manganese dioxide, as it is called, also yields one-third of its oxygen on being stroiigly heated. The equation is 3 MnO 2 heated L MaX, + o! manganese dioxide = magnetic oxidl ol manganese + O. The atomic weight of manganese is 55 ; therefore, as seen by the equation, 2611b of the dioxide will yield 321b of oxygen. Chlorate of potash, X 010 3 yields all its oxygen on being strongly heated. 3 Lloß f a rr d - ?^ ,+, + °* Th e atomiclweight wilfyie^^ Of •**■£• Perhaps the easiest way of getting a good supply of oxygen for experiments is to heat in a suitable retort a mixture of 6 or 8 parts of the chlorate with 1 part of either manganese dioxide or black oxide of copper. The oxygen in this case comes off at a lower tempeVaKhiSS C °T efxcloßivele f xcloßivelv fr °m the chlorate ; the black oxide of manganese or of copper remaining the same all through the experiment # Another way pf getting a current of oxygen is to mix some chloride of cobalt with a strong solution of chloride of lime, and heat the mil ture to 80 degrees Centigrade. Sulphuric acid, when heated strongly as by passing its vapour through a red-hot porcelaij or platinum tube, gives off oxygen thusa 1 t. . H2SO-l=H 2 S O-l=H 2+SOo+o2 +SOo+o Sulphuric acid = water + sulphurous acid + oxygen. ' m Lastly, oxide of barum, BaO, heated dull red in the air, assumes another atom of oxv»en ha coming peroxide , BaO* and this compound heated in a retort red hot, gives up one-half its oxygen, becoming again BaO. By the orocefa of alternately heating dull red and bright red it becomes a carrier of oxygen from the air. The properties of oxygen were then illustrate?. A splinter of wood was inflamed, blown out and the rea-tippad charred end plunged into a jar of oxygen. It burst into brilliant combustion, blazingand crackling and scintillating with dazzling light. This was explained asan instance of intense or vigorous combustion or oxidatioi) the carbon of the splinter unitins vinlAnfl^ with the oxygen to form C 0. 2 cSfonfc 3 dnde, " s «ally termed carbonic acid, 121b of charcoal for 441b of the acid, as seen by the equation : * bUB .. C+O 2 =CO 2 A piece of phosphorus was then rapidly dried and ignited over water, and then ah 2 globe of oxygen was inverted over it, 80 that the phosphorus was in the midst of the oxygen. Immediately huge volumes of a snowwhite, cloudy flaky solid filled the globe through which was seen the bright light of the phosphorus burning the whole globe appearir? as a large ball of light. The white cloudy soli? was explained to be dry phosphoric acid-or more correctly, phosphoric anhydride,~the equation being— * ' tuo P 2 +O 5=5 =P2P 22 O5O 5 The atomic weight of P is 31, sixty-two parts of phosphorus yielding 142 parts of the anbvdnde. J Some iron wire worked into a spiral coll was noxt tipped with sulphur, lighted, and pluntred into a jar of oxygeii. At first the sulphur was seen to burn brilliantly, with a rosy lilac tint and then the iron ignited and burned with a dazzling white light the burning and scin. tillatmg point rapidly rising a 8 the iron was oxidised, and dropped away in a melted state. The equations shown were— go c il.S+ 2? * =s °2 [anhydride 32 of sulphur + 32 of oxygen = 64 of sulphurous Fe 3 -f- 1 z=z F e 0 168 of iron +64 of oxygen = 232 3 of magnetic of iron, liko the loarJsfcone or our black ironsand, the atomic weight of iron being 56. In this connection ft jar was filled two-thirds with hydiogen and one-third oxygen, a naked light was introduced into the mixture, and the gases exploded with a loud report : the equations being— H2H 2 + O=H 2 O 2 parts of H+l6 parts of o=lß parts of water. Saltpetre, or the nitrate of potassium, and many other nitrates are also good sources of inapura oxygen when heated. This was shown by heating saltpetre iv a glass tube till it fused ; Home powdered charcoal was then thrown into it, nn.-i i,ha heat continued. Very soon the charcoal ignUotl and burned with a strong violet light, the heat being so great as to melfc

the glass tube. The violet colour of the light was explained to be duo to small parts of the potassium getting burned along with the charAs the time had nearly expired, the Professor intimated that he would finish the study of oxygen and its interesting allotropic form, ozone, in next lecture. The class then split into two sections, one remaining in the lecture-room with MrCattan, the other going with the Professor into the laboratary. This division was made owing to want of room for the large number present. Both divisions were then taken through the tests for eolutions of metallic salts. The metals operated on were silver (A.g) lead (Pb), and mercury protoselts (Hg'). The salts selected were the nitrates of these metals, Ac' No 3 Pb' (No 3 V 2 and Hg' No, respectively, all being in solution. Three drops of dilute hydrochloric acid (muriatic acid) were put into %each solution, when instantly they all three thrown down in a white precipitate. It was pointed out that among the metals silver, lead, and mercury (proto) are the only ones precipitated by this acid. Over each white precipitate was then poured some ammonia solution NH 3 , when, instantly on being shaken, the silver white dissolved and cleared up, the lead white remained unchanged, and the mercury white was blackened. This showed how, by means of muriatic acid and ammpnia, Bilver, lead, and proto-mercury can be distinguished not only from all the other metals, but also from one and another. As another confirming test, a solution of bichromate of potash (the chrome of druggists), K3OK 3 0r 5 0 7 was poured into a fresh portion of each metallic solution, whereupon the silver was thrown down dark red, the lead bright yellow, and the proto-mercury orange-coloured. Caustic potash solution, KHO, was then introduced into another portion of each metallic solution, when the silver was thrown down grey, the lead at first white, but on addition of more potash the white disappeared, and the mercury black. , lodide of potash, XI, was then added to a fresh sample of each, when the silver was precipitated yellow-white, the lead yellow, and the mercury dirty green. Every student present, to the number of 100 at least, performed individually these tests, and noted carefully the conditions and the results of the experiments. , , A , Mr Oattan— one of the beat students of the Chemistry class and laboratory last seßsion— has kindly undertaken the Ipart so ably performed by Mr Reid in the Saturday course! of last winter. . The subject of next lecture is ozone and nitrogen; land the metals to be tested are copper, cadmium, gold, and arsenic.

Name. Symbol. Atomic Weight. Atomic Volume] Hydrogen „ IF 1 n 0 Oxj'gcn ■0" 1G Sulphur , Selenium Tellurium Fluorine Chlorine S" So" .Te" ,F,, F , 01' Br' 32 79 128 19 35-5 80 0 0 0 n 0 0 0 0 o n Bromine lodine X 127 Carbon G"" 12 Boron B'" 11 Silicon .Si"" 28 Nitrogen ' >" 14 0 Phosphorus Vp;// 31 V