ON TEACHING CUEMISTRT. 239 



carbonaceous substances is therefore tested for by breathing into lime 

 water. The discovery thus made affords an opportunity for a digression 

 and for explaining how plants derive their carbon from the air. 



PROBiiE.y VI. To determine what happens when sulphur is burnt. — From, 

 the results of the experiments with carbon, it appears probable that the 

 disappearance of sulphur when burnt is also really due to its conversion 

 into a gaseous oxide, so it is kindled and introduced into oxygen : if it be 

 burnt over water in a bell jar in a spoon passing through the stopper (a 

 rubber cork), the water is seen to rise ; if, on the other hand, it be burnt 

 in a dry flask closed by a rubber cork carrying a gauge-tube, as suggested 

 by Hofmann,' the volume is seen to be almost unchanged after combus- 

 tion. It follows, therefore, that the sulphur and oxygen unite and form 

 a soluble product. Sulphur is next burnt in a tube in a current of oxy- 

 gen, and the gas is passed into water ; a solution is thus obtained having 

 the odour of the gas and sour (acid) to the taste. The fact that carbon 

 and sulphur — both non-metals — behave alike in yielding gaseous oxides 

 suggests that a comparison be made of their oxides : so the acid solution 

 is added to lime water ; a precipitate is formed, which rsdissolves on 

 adding more of the sulphur gas solution ; on the other hand, on add- 

 ing the lime water to the acid liquid, this latter after a time loses its 

 characteristic smell. There can be no doubt, therefore, that the sulphur 

 gas does in some way act upon the lime. The discovery that the addition 

 of more of the sulphur oxide leads to the dissolution of the precipitate 

 which it first forms in lime water suggests trying the effect of excess of 

 the carbon oxide on the lime water precipitate ; this is done, and the dis- 

 covery is made that the precipitate gradually dissolves. The solubility 

 of the new substance may then be determined by passing the gas into 

 water containing chalk in suspension, filtering, and evaporating. This 

 leads to the observation that a precipitate is formed on heating the liquid, 

 and this is soon found to be chalk. An opportunity is thus afforded of 

 explaining the presence of so much ' chalk ' in water ; of demonstrating 

 its removal by boiling and by lime water ; and the effect it has on soap. 



The observation that the oxides of both carbon and sulphur combine 

 with lime suggests trying whether the one will turn out the other, so 

 the solution of the sulphur oxide is poured on to chalk : effervescence is 

 observed, and on passing the gas into lime water a precipitate is obtained. 

 The production of this effect by the acid solution suggests trying common 

 vinegar — a well-known acid substance. This also is found to liberate 

 chalk gas, and the discovery of an easy method of preparing chalk gas is 

 thus made. The oxide formed on burning phosphorus, having previously 

 been found to give an acid solution, is tried, and it is found that it also 

 liberates chalk gas. As a good deal of vinegar is found to give very 

 little chalk gas, the question arises. Are there not acids to be bought 

 which will have the same effect and are stronger and cheaper? On 

 incjuiry it is found that sulphuric acid or oil of vitriol, muriatic acid or 

 spirits of salts, and nitric acid or aquafortis may be bought, and that these 

 all act on chalk. The behaviour of chalk with acids affords a means of 

 testing the lime water precipitate obtained in working out Problems IV. 

 and V. In this manner the pupil is led to realise that certain agents may 



' iJy burning carbon also in this way a most effective demonstration is given of 

 the fact that no loss or gain of matter attends the change, and that only heat 

 escapes ; the results in the case of carbon and sulphur are particularly striking, as 

 the products are gaseous and invisible. 



