6o4 



NATURE 



\Oct. 17, 1889 



be colourless, and to become coloured on coming into contact 

 with air— oxygen and nitrogen are, therefore, added to portions 

 of the gas over water. In this manner, not only is a new gas 

 discovered, but also a test for oxygen ; and opportunity is 

 afforded of here calling attention to the fact that air behaves 

 exactly as oxygen, that the oxygen in air appears to be unaffected 

 by its association with nitrogen — that, in fact, it is uncombined. 

 From these experiments it is obvious that melals and acids inter- 

 act in a variety of ways. Finally, the dissolution of gold and 

 platinum by aqua regia may be demonstrated. 



Problem VIIL To deter?>nne what happens when oxides are 

 treated with acids. — In the course of the previous experiments 

 a number of oxides have been prepared by burning various 

 metals in air ; these are found to be unchanged by water. The 

 discovery that acids act on metals suggests a trial of the effect 

 which acids will have on their oxides ; so the oxides of zinc, 

 iron, copper, and lead are submitted to the action of the three 

 acids previously used. Sulphuric acid is found to dissolve zinc 

 oxide, iron rust, and copper oxide, but no gas is evolved ; excess 

 of the oxide may be used, and the filtered liquid concentrated ; 

 the crystals which separate may be examined and compared 

 with those obtained by dissolving the metal in sulphuric acid, 

 &c. Litharge apparently is not changed by sulphuric acid, but 

 red lead is, although not dissolved. Muriatic acid being used, 

 all the oxides are found to dissolve, and in the case of red 

 lead a greenish-yellow gas is given off possessing a most dis- I 

 agreeable smell : this is noted as a case for study. The product 

 from the lead oxides is observed to crystallize out from the hot 

 liquid on standing, so the undissolved original product is boiled 

 up with water, and the solution is filtered, &c. Attention is thus 

 directed to the difference in solubility of the products. Next, 

 aquafortis is used ; again all are dissolved, except the red lead, 

 which, however, is obviously altered. In the case of the lead 

 oxides the product is again less soluble than those afforded by 

 the other oxides, but more soluble than the product obtained on 

 using muriatic acid. The pupil has already been led to realize 

 that of two substances capable of acting on a third, such as chalk- 

 gas and sulphur gas, which both combine with lime, one may be 

 the stronger, and may turn out the other, sulphur gas turning 

 out chalk gas from chalk. A comparison of the three acids with 

 the object of ascertaining which is the strongest is therefore 

 suggested : the melal or oxide is dissolved in one of the acids, 

 and the others are then added. No positive result is obtained in 

 case of zinc, iron, or copper, but the solution of lead in nitric 

 acid is precipitated by muriatic and by sulphuric acid ; the former 

 precipitate is found to dissolve in boiling water and to crystallize 

 out in exactly the same way as the substance obtained from lead 

 oxide and muriatic acid. The sulphuric acid product is found to 

 be almost insoluble in water, and also in muriatic and nitric 

 acids : these observations make it possible, by examining the 

 behaviour towards muriatic and nitric acids of the products of the 

 action of sulphuric acid on the lead oxides, to establish the fact 

 that the product is the same whether lead be dissolved in nitric 

 acid and sulphuric acid be then added, or whether either of the 

 oxides be treated with sulphuric acid. It is further evident that 

 those acids which give difficultly soluble or insoluble products 

 act with difficulty if at all on the metal. Other metals besides 

 those mentioned may be now studied, and, a solvent being found, 

 the acids which do not dissolve the metal may be added to the 

 solution. In this way, for example, the chloride test for silver 

 is discovered. 



In experimenting with acids the pupils can hardly fail to stain 

 their clothes and their fingers. The observation that acids alter 

 colours serves to suggest experiments on the action of acids on 

 colours, especially those of leaves and flowers. The use of 

 litmus, methyl-orange, cochineal, &c., may then be explained. 

 As various oxides have been found to "neutralize" acids, the 

 study of their effect on the colours altered by acids is suggested. 

 Lastly, a few experiments with vegetable and animal substances, 

 sugar, &c. , may be made, which demonstrate the corrosive 

 action of oil of vitriol and aquafortis. 



Problem IX. To determine what happens when the gas ob- 

 tained by dissolving iron or zinc in sulphuric or mwiatic acid is 

 burnt. — The gas has been observed to burn with a smokeless, 

 odourless flame. To ascertain whether, as in all other cases of 

 combustion previously studied, the oxygen of the air is con- 

 cerned in the combustion, a burning jtt of the gas is plunged 

 into a dry cylinder full of oxygen, in which it is not only 

 seen to continue burning, but it is also noticed that drops of 

 liquid condense on the cylinder above the flame ; this im- 



mediately suggests that the product is a liquid. The jet is found 

 to be extinguished in nitrogen, so evidently when the gas burns 

 it forms an oxide. The experiment is repeated, and the gas 

 burnt in a bell jar full of oxygen over water : the water rises- 

 as the combustion proceeds, proving that the oxygen is used up. 

 To collect a sufficient quantity of the product for examination, 

 the dried ^ gas is burnt at a jet underneath a Florence flask 

 through which a stream of cold water is allowed to circulate : 

 the neck of the flask is passed through the neck of a bell jar and 

 the flask and bell jar are clamped up in an inclined position, so- 

 that the liquid which condenses may drop into a small beaker 

 placed below the rim of the jar. What is the liquid ? It looks 

 very like water, and is without taste or smell. Is it water ? 

 How is this to be ascertained ? What are the properties of 

 water ? The knowledge previously gained here becomes of 

 importance. It has been observed that frozen water melts at 

 0° C, that water boils at 100°, and that one cubic centimetre 

 weighs one gramme at 4° C. ; so the liquid is frozen by the ice- 

 maker's mixture of ice and salt, a thermometer being plunged 

 into it so that the solid ice forms on the bulb : the melting-point 

 is then observed. Subsequently the boiling-point is determined, 

 a little cotton-wool being wrapped around the bulb of the 

 thermometer. Lastly, the density of the liquid may be deter- 

 mined. It is thus established that the gas yields water whei> 

 burnt, and the name of the gas may now for the first time be 

 mentioned and explained. The results thus obtained leave little 

 doubt that water is an oxide of hydrogen ; but in order to place 

 this beyond doubt it is necessary to exclude nitrogen altogether. 

 How is this to be done ? Red lead is known to consist of lead 

 and oxygen only, and readily parts with a portion at least of its- 

 oxygen ; so dried oxygen is passed over red lead, which is then 

 gently heated. Again a liquid is obtained which behaves as 

 water, so there can be no doubt that water is an oxide of hydro- 

 gen. Water is not obtained on merely mixing oxygen and 

 hydrogen ; it is only produced when combustion takes place. 

 To start the combustion a flame is applied to a small quantity 

 of a mixture of the two gases : a violent explosion takes place. 

 An opportunity is here again afforded of calling attention tO' 

 the entire change in properties which takes place when the com- 

 pound is formed. On heating red lead in hydrogen, lead is 

 obtained, although on heating it alone it loses only a portion of 

 its oxygen, and the " reduction " takes place very readily ; 

 evidently, therefore, hydrogen is a powerful agent. This obser- 

 vation suggests further experiments. Will it not be possible to- 

 remove oxygen by means of hydrogen from other oxides which 

 are not altered on heating? and will not other combustible 

 substances besides hydrogen remove oxygen from oxides ? 



Problem X. To determine what happens when hydrogen and 

 other combustible substances are heated with oxides. — -Zinc oxide, 

 iron rust, and copper oxide are now heated in a current of hydro ■ 

 gen : the first remains unaltered, the other two are seen to 

 change, a liquid being formed which it cannot be doubted is 

 water ; the copper oxide evidently becomes reduced to copper. 

 Is the iron rust similarly reduced to the metallic state? How 

 is iron to be tested for? Iron is attracted by the magnet, and 

 dissolves in diluted oil of vitriol with evolution of hydrogen. 

 Applying these tests, no doubt remains that the iron rust is 

 deprived of its oxygen. 



Litharge and copper oxide may then be mixed with soot or 

 finely powdered charcoal and heated in tubes ; gas is given off 

 which renders lime water turbid, and metallic lead or copper is 

 obviously obtained. It is thus established that some but not all 

 oxides may be deprived of their oxygen by means either of 

 hydrogen or carbon. Opportunity is here afforded of explaining 

 the manufacture of iron. 



Several dried combustible organic substances, sugar, bread, 

 and meat, may now be burnt with copper oxide in a tube the 

 fore part of which is clean and is kept cool : liquid is seen to 

 condense, while "chalk gas" is given off; the liquid has the 

 appearance of water, and sufficient may easily be obtained to 

 ascertain whether it is water. The presence of hydrogen in 

 organic substances is thus discovered ; its origin from water may 

 now be explained, and the double function of water in the plant 

 and animal economy may be referred to — viz. that it both enters 

 into the composition of the animal and plant structure and also 

 acts as a solvent. The combustion of ordinary coal gas, of 

 alcohol, of petroleum, of oil and of candles, may then be 



' The importance of cryhg the gas is realized without difficulty, as- 

 previous observations have shown that the air is moist, and as- the gas is- 

 given off in presence of water, lime may be used. 



