no 



NA TURE 



[June i, 1882 



CHEMICAL LECTURE EXPERIMENTS 



SEVERAL interesting and instructive experiments have 

 been described of late in the chemical journals : we 

 propose to give a short account of the more important. 



I.— Physico-chemical Experiments 



A. Mixing of Gases by Diffusion. — That gases do not 

 mix very rapidly by diffusion alone, may be illustrated by 

 placing a strip of white paper moistened with lead acetate 

 solution inside a tall glass stoppered cylinder, so that 

 when the cylinder is inverted the paper extends from the 

 bottom (which now forms the uppermost end) not more 

 than one-third of the total length of the cylinder. A little 

 sulphuretted hydrogen water is placed in the hollow stopper 

 of the cylinder, and the stopper is inserted loosely into its 

 place. After ten to fifteen minutes the production of 

 brown lead sulphide on the white paper shows that the sul- 

 phuretted hydrogen gas has risen two- thirds of the height 

 of the cylinder. Paper moistened with starch and potas- 

 sium iodide, and chlorine water, may respectively replace 

 the lead acetate paper and the sulphuretted hydrogen 

 water (C. von Than, Berichte, xii. 14 14). 



B. Liquefaction of Gases by Increasing Pressure or 

 Lowering Temperature. — Ethylic chloride, which boils at 

 12', is easily liquefied. A Hofmann's lecture eudiometer, 

 with one limb about 100 cm. and the other 50 cm. in 

 length, serves as apparatus. The eudiometer is filled 

 with mercury ; ethylic chloride gas is led into the shorter 

 limb through the upper stopcock, while the mercury flows 

 out by the other stopcock ; when the smaller limb is partlv 

 filled with gas, the mercury is adjusted to the same level 

 in both limbs, the gas is liquefied by pouring ether over 

 the shorter limb, and any air which has entered with the 

 gas is allowed to escape by opening the upper stopcock 

 for a moment. The liquid ethylic chloride is gasified by 

 running out as much as possible of the mercury in the 

 larger limb, and so reducing the pressure. If the tem- 

 perature is lowered— by pouring ether on to the outside of 

 the shorter limb— or if the pressure is increased— by 

 pouring mercury into the longer limb, equal to an extra 

 half atmosphere of pressure— the ethylic chloride becomes 

 liquid (Hofmann, Berichte, xii. 1124). 



C. Absorption of Heat Accompanying Change from 

 Liquid to Gaseous Form.— A test tube, partly filled' with 

 water, is placed in a small glass cylinder containing ether 

 sufficient to completely cover that part of the tube in 

 which there is water. A brisk stream of dry air is driven 

 through the ether, which rapidly evaporates; in a few 

 minutes the water in the tube is completely frozen (Hof- 

 mann, Berichte, xii. 1125). 



D. Thermal Changes Attending Solution of Salts in 

 Water. — A small flask of about 100 cc. capacity is fitted 

 with a cork carrying a glass tube, 3-4 mm. diameter, bent 

 twice at right angles ; the larger limb of the tube is about 

 70 cm. in length, and passes through a cork nearly to the 

 bottom of a wide-mouthed bottle, of about 40 cc. capacity, 

 containing coloured water. A straight piece of similar 

 tubing of about the same length passes through a second 

 hole in the cork, and also reaches nearly to the bottom of 

 the wide-mouthed bottle. The salt under examination is 

 placed in the flask, water is added, and the cork is in- 

 serted. If heat is evolved during solution, the coloured 

 water rises in the straight glass tube, which is open at 

 the upper end ; if heat is absorbed during solution, the 

 water rises in the tube connected with the small flask 

 (Rosenfeld, Berichte, xiii. 1475). 



II.— Experiments. Illustrative of Chemical 

 Action in General 



A. Conservation of Mass.— Two small glass tubes, about 

 15 cm. long by 2 cm. wide, are sealed and rounded off at 

 one end and drawn out at the other to tubes about 3 mm. 

 diameter. About two centigrams of freshly heated char- 



coal is dropped into one tube, the air is replaced by dry 

 oxygen, which is led in by a capillary tube passing through 

 the narrow opening, and the tube is sealed. The other 

 tube is also sealed at a point such that the weights of the 

 two tubes are equal. The tubes are placed on the oppo- 

 site pans of a balance, and the balance is shown to be 

 in equilibrium : that tube which contains charcoal is 

 heated by a small gas-flame, the charcoal burns brilliantly, 

 and by carefully shaking the tube is all, or almost all, 

 consumed. When the tube is cold it is replaced on the 

 balance pan, on releasing the beam it is found that no 

 change has occurred in the mass of matter in the tube, 

 although the form of the matter has undergone most 

 marked change. Sulphur, or a very small quantity of 

 gun-cotton, may be employed in place of charcoal in this 

 experiment (C. von Than, Berichte, xii. 1413). 



B. The Individual Substances taking part in a Chemical 

 Change Gain or Lose Weight. — To demonstrate that a 

 substance increases in weight during oxidation, &c, or 

 loses weight during reduction, &c, a piece of copper- 

 wire, about 10 cm. long by 1 mm. thick, is fused into 

 the upper end of a glass hydrometer: the wire carries a 

 little glass cup (the bottom part of a test tube serves ad- 

 mirably) on its upper end, on which lies a piece of plati- 

 num foil. The hydrometer is placed in water contained in 

 a cylinder. Such a quantity of finely divided iron as suffices 

 to sink the hydrometer, so that the wire is just wetted, is 

 placed on the platinum foil ; the foil is removed, heated 

 till the iron is oxidised, allowed to cool, and replaced on 

 its support : the hydrometer sinks considerably in the 

 water. To illustrate loss of weight on reduction, a little 

 cylinder of copper oxide, made by mixing the substance 

 with gum and drying, may be employed. Before the re- 

 duction is commenced, nearly the whole of the wire sup- 

 porting the platinum foil ought to be immersed in the 

 water (Rosenfeld, Berichte, xiv. 2102). 



C. Influence of Mass, Time, etc., on a Chemical Change. 

 — The influence of time, temperature, and mass, as also 

 the meaning of the phrase reverse action, may be quali- 

 tatively illustrated as follows : — Three beakers are ar- 

 ranged on white surfaces and with white backgrounds ; 

 in beaker (1) is placed about 100 cc. of cold water, in (2) 

 the same quantity of water at cjo'-ioo , and in (3) about 

 500 cc. of cold water. A few drops of a solution of bis- 

 muth iodide in concentrated hydriodic acid is poured into 

 each beaker ; brown bismuth iodide is precipitated in the 

 first beaker, red bismuth oxyiodide in the second, and the 

 same salt, but in smaller quantity, in the third. On stand- 

 ing for a little time, the brown iodide is slowly changed 

 into red oxyiodide ; but on adding a little concentrated 

 hydriodic acid, the reverse change — viz. from oxyiodide 

 to iodide — takes place (Muir, Chem. Soc. Journ. Trans., 

 1882, 6). 



The influence of mass and time on a chemical change 

 may be quantitatively exhibited by arranging a series of 

 similar glass cylinders filled with water, and inverted in 

 glass basins : a stoppered retort of about 100 cc. capacity 

 is supported close to each cylinder. The retorts are care- 

 fully cleaned, and 50 cc. of pure sulphuric acid, regularly 

 diminishing in concentration by a fixed amount, is placed 

 in each. Sheet zinc is cut into squares of equal sizes, 

 which are rolled into slit cylinders (by bending round a 

 glass rod), cleansed in soda solution, then washed, im- 

 mersed in strong sulphuric acid, again rapidly washed in 

 a stream of water, and dropped into the retorts, which 

 are then stoppered. The hydrogen which is evolved is 

 collected as long as the areas of the zinc remain visibly 

 constant. The upper surfaces of the water in the cylin- 

 ders exhibit, in the form of a curve, the influence of the 

 mass of sulphuric acid on the change under consideration. 

 As the evolution of hydrogen may be stopped at any 

 moment by withdrawing the stopper of a retort, the ex- 

 periment may be arranged to show the influence of time 

 on the change (Mills, Chem. Soc. Journ. Trans., 1S80, 454). 



