ON TEACHINa CUEMISTBT. 



Stage V. — The quantitative stage. 



247 



The quantitative composition of many of the substances which have 

 previously been studied qualitatively should now bo determined — in some 

 cases by the teacher in face of the pupils, so that every detail may be 

 observed and all the results recorded ; in other cases by the pupils. 



The composition of water is first determined by Dumas' method ; this 

 may easily be done, and fairly accurate results may be obtained in tho 

 course of a couple of hours. The results obtained by Dumas and subse- 

 quent workers should then all be cited, and, attention having been drawn 

 to the extent to which such experiments are necessarily subject to error, 

 the evidence which the results afford that hydrogen and oxygen combine 

 in certain fixed and invariable jjro;portions to form water is especially 

 insisted upon. 



The composition of chalk gas is next determined; this also is easily 

 done, as impure carbon (lampblack) may be burnt and the hydrogen 

 allowed for. Again, attention is directed to the results obtained by 

 skilled workers, and the evidence which they afford that chalk gas never 

 varies in composition. 



The composition of copper oxide has already been ascertained ; it may 

 be re-determined by reducing the oxide in hydrogen : in fact, m deter- 

 mining the composition of water. 



The lead oxides may be reduced in a similar manner, the oxide 

 obtained by ifjniting white lead as well as red lead and the brown oxide 

 obtained by a'cting on red lead with nitric acid being used. In this way 

 it is ascertained that the brown oxide is the highest oxide; the loss m 

 weight which this oxide suffers when ignited may then be determined. 



Tabulating the results thus obtained, after calculating with what 

 amount of the particular element that quantity of oxygen is associated 

 which in water is combined with one part by weight (tinit weight) of 

 hydrogen, numbers such as the following are obtained : — 



1 part of hydrogen is combined with 8 parts of oxj-gen in water 

 3 „ carbon „ „ 8 „ „ chalk gas 



31-5 „ copper „ „ 8 „ „ copper oxide 



10,3-5 „ lead „ „ 8 „ ,. lead oxide (litharge) 



61-8 , „ 8 „ „ „ (brown) 



These clearly illustrate the fact that elements combine in very different 

 proportions, and the results obtained with the lead oxides afford also an 

 illustration of combination in multiple proportion. 



The amounts of silver and lead nitrates formed on dissolving silver and 

 lead in nitric acid are next determined by evaporating the solutions of 

 known weights of the metals in porcelain crucibles on the water-bath, and 

 then drying until the weight is constant ; accurate results may bo easily 

 obtained, and these two exercises afford most valuable training. The 

 nitrates are subsequently evaporated with muriatic acid and the weights 

 of the products determined. What are these products? Does the metal 

 simply take the place of the hydrogen in hydrogen chloride as zinc does 

 when it dissolves in muriatic acid ? If so, the products are silver and 

 lead chlorides, and it may be expected that the same substances will bo 

 obtained— that the same increase in weight will be observed, when, say, 

 silver is combined directly with chlorine as when it is dissolved in nitric 

 acid and the solution is precipitated with muriatic acid or salt. Silver 



