234 EBPOET-1889. 



charcoal, for instance — there may nevertheless be a product. Whereas, 

 therefore, in Stage I. the pnpil will have learnt to appreciate the exist- 

 ence of a great variety of substances, and will have gained the power of 

 describing their outward appearance more or less fully ; and in Stage II., 

 having learnt how to measure and weigh, will acquire the habit of deter- 

 mining by measurement certain properties of substances, and will thus be 

 in a position to express in exact terms the kind of differences observed ; 

 in Stage III. the pupil will be led to see that profound changes take place 

 on burning substances, and that these changes involve something more 

 than the destruction of the things burnt. The foundation is thus laid for 

 the study of change, i.e., chemical studies proper. 



Stage IV. — The prohlem stage. 



Many of the changes observed in the course of the experiments made 

 in Stage III. might be examined and their nature determined, but the 

 best to take first is a very familiar case, that of the rusting of iron. 



Problem I. To determine what happens lohen iron rusts. — The pupil 

 w,ust be led in the first instance to realise that a problem is to be solved 

 and that the detective's method must be adopted and a chie sought 

 for. It is a familiar observation that iron rusts, especially when wet ; 

 what happens to the iron, why does it rust, is the iron alone concerned in 

 the change ? No information can be gained by looking at it — perhaps the 

 balance which has brought to light so much in Stage III. may be of ser- 

 vice, so the iron is allowed to rust in such a manner that any change in 

 weight can be observed. A few grams of iron-filings or borings are put 

 on to a weighed saucer or clock glass along with a bit of stiff brass or 

 copper wire to be used as a stirrer ; the iron is weighed, then moistened 

 and exposed under a paper cover to keep off dust, preferably in a 

 warm place ; it is kept moist and occasionally stirred. After a few days 

 it is dried in the oven and then weighed. The weight is greater. Some- 

 thing from somewhere has been added to the iron. Thus the clue is gained. 

 Where did this something come from ? The fact that when a tumbler, 

 for instance, is plunged mouth downwards into water the water does not 

 enter, and that on gradually tilting the tumbler to one side something 

 escapes — viz., air — at once affords a demonstration of the presence of 

 air in the space around us. The iron rusted in this air, but was kept 

 moist, so it may have taken up the something from either the air or the 

 water. To ascertain whether the air takes part in the rusting, some iron 

 borings are tied up in a bit of muslin and the bag is hung from a wire 

 stand placed in a (jam) pot full of water and a so-called empty (pickle) 

 bottle, which in reality is full of air, is inverted over the iron ; in the 

 course of a few hours, as the iron rusta, the water is observed to rise 

 until it occupies about one-fifth of the jar (determined by measuring or 

 weighing the water) ; the something added to the iron during rusting 

 appears therefore to come from the air, and the all-important fact is thus 

 discovered that the rusting is a change in which not the iron alone, but 

 also the air, is concerned. The experiment is several times repeated, 

 fresh iron being used with the same air and the same iron put in suc- 

 cession into fresh portions of air, but the same result is always obtained : 

 whence it follows that whatever it is in the air which takes part in the 

 rusting, the air as a whole is not active. The changes previously ob- 



