ON TEACHING CHEMISTRY. 241 



disagreeable smell ; this is noted as a case for study. The product from 

 the lead oxides is observed to crystallise 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 differ- 

 ence 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 realise 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 metal or oxide is dissolved in 

 one of the acids, and the others are then added. No positive result is 

 obtained in the 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 crystallise 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 dis.solved 

 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 whicli give difiBcultly soluble or in.soluble 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, methylorange, cochineal, &c., 

 may then be explained. As various oxides have been fonnd to ' neu- 

 tralise ' acids, the study of their effect on the colours altered by acids is 

 suggested. Lastly, a few experiments with vegetable and animal sub- 

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

 of oil of vitriol and aquafortis. 



Problkm IX. To determine tvhat Jiappens when the gas ohtained by 

 dissolvir.g iron or zinc in siilplniric or 'inuriatic acid is hurnt. — 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 concerned in the combustion, a burning jet 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 immediately suggests 

 that the product is a liquid. The jet is found to be extinguished in nitro- 

 gen, 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 



1889. R ' 



