Oct. I J, 1889] 



NATURE 



601 



may be used to examine melting ice and boiling water ; the con- 

 struction of both the Centigrade and Fahrenheit thermometer 

 ■may then be explained, and the effect of heat on bodies made 

 clear. The density of ice and of water at various temperatures 

 -may then be determined, a Sprengel tube — which is easily made 

 — being used for warm water ; the bursting of pipes in winter, 

 the formation of ice on the surface of water, &c., may then be 

 explained. Afterwards simple determinations of the heat capacity 

 of a few metals, &c., and of the latent heat of water and steam, 

 may be made in accordance with the directions given in a book 

 such as Worthington's "Practical Physics." 



Stage III. — Sfiidies of the effect of heat on things generally ; 

 of their behaviour when burnt. 



As it is a matter of common observation that heat alters most 

 things, the effects of heat on things generally should be studied ; 

 in the first instance qualitatively, but subsequently, and as early 

 as possible, quantitatively. Bits of the common metals may be 

 heated in the bowl of an ordinary clay pipe plunged into a clear 

 place in any ordinary fire, or in such a pipe or a small iron spoon 

 ■over a gas flame. 1 he difference in fusibility is at once apparent, 

 and in the case of metals like iron and copper it is noticeable 

 that although fusion does not take place, a superficial change is 

 produced ; the gradual formation of a skin on the surface of 

 fused lead and tin is also easily perceived. Observations like 

 this become of great importance at a later stage, and indeed 

 serve to suggest further experiments : this is a point of special 

 importance, and from the beginning of this stage great attention 

 should be paid to inculcating habits of correct observation ; the 

 •effect should first be recorded by the pupil, the notes should then 

 be discussed and their incompleteness pointed out, and they 

 should afterwards be re-written. The fusibility of substances 

 which are not affected when heated in the tobacco pipe may be 

 tested by heating them with a Fletcher gas blow-pipe on charcoal; 

 and by heating little bits of wire or foil in such a flame it is easy 

 for children to discover the changes which metals undergo when 

 burnt, especially in cases such as that of zinc or copjier or iron. 

 The further study of the effects of heat should be quantitative, 

 and may well commence with water. It being observed that 

 water disappears on heating, water may be put into a clock glass 

 or glass dish placed on a water bath (small saucepan) ; it evapo- 

 rates, and it is then observed that something is left. A known 

 quantity of water by weight or volume is therefore evaporated 

 and the residue weighed. This leads to the discovery that water 

 contains something in solution. The question then naturally 

 arises, What about the water that escapes ? so the steam is con- 

 densed and the distilled water evaporated. The conception of 

 pure water is thus acquired. An experiment or two on dissolution 

 — using salt and sugar— may then be introduced, a water oven or 

 even an air oven (a small Fletcher oven) kept at a known tem- 

 perature being used, and the residue dried until the weight is 

 constant. Rain and sea-water may next be examined ; the 

 results afford an opportunity of explaining the origin of rain and 

 of accounting for the presence of such a large quantity of dis- 

 solved matter in sea-water. Then the various common food 

 materials may be systematically studied, commencing with milk ; 

 they should first be dried in the oven, then carbonized and the 

 amount of char determined, then burnt and the percentage of 

 ashes determined. A small platinum dish, 15 to 20 grammes in 

 weight, is required for these experiments, and a gas muffle 

 furnace is of the greatest use in burning the char and in oxidizing 

 metals. In addition to the discipline afforded by such experi- 

 ments a large amount of valuable information is acquired, and 

 the all-important fact is established that food materials generally 

 are combustible substances. Afterwards mineral substances are 

 examined in a similar manner, such as sand, clay, chalk, sulphur, 

 &c., and then inetals such as lead, copper, tin, and iron may be 

 studied ; their increase in weight is in striking contrast to the 

 inalterability of substances like sand and ualt, and the destruction 

 of vegetable and animal substances. Chalk, from which lime is 

 made by burning, is found to occupy a middle position, losing 

 somewhat in weight when strongly heated. The exceptional 

 behaviour of coal among mineral substances, and of salt among 

 food materials, is shown to be capable of explanation, inasmuch 

 as coal is in reality a vegetable and salt a mineral substance ; but 

 sulphur remains an instane of exceptional behaviour requiring 

 explanation. It is not exceptional in being combustible, as 

 metals like magnesium and zinc are combustible, but in affording 

 no visible product. The smell of burning sulphur, however, 

 serves to suggest that perhaps, after all, there is a something 



formed which is an invisible substance possessed of an odour, 

 and then follows quite naturally the suggestion that perhaps in 

 other cases where no visible or perceptible product is obtained — 

 as on burning charcoal, for instance — there may nevertheless be 

 a product. Whereas, therefore, in Stage I. the pupil will have 

 learnt to appreciate the existence of a great variety of substances, 

 and will have gained the power of describini^ their outward 

 appearance more or less fully ; and in Stage II. having learnt 

 how to measure and weigh, will acquire the habit of determining 

 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 W .—The problem stage. 



Many of the changes observed in the course of the experi- 

 ments mcde 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 when iron rusts. — 

 The pupil must be led in the first instance to realize that a 

 problem is to be solved, and that the detective's method must be 

 adopted and a clue 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 

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

 change in weight can be observed. A few grammes 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. Something 

 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 reaUty is full of air, is inverted 

 over the iron ; in the course of a few hours, as the iron rusts, 

 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-:mportant fact is thus dis- 

 covered 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 succession 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 observed to take place 

 when iron, copper, lead, zinc, &c. , were heated in air, are then 

 recalled ; as the metals were found to increase in weight, it 

 would appear probable that in these cases of change also the air 

 was concerned. 



These results at once suggest the question, What is air ? So 

 much having been learnt by studying the change which iron 

 undergoes in rusting, other changes which happen in air there- 

 fore are next studied. 



Problem II. To detn-mine the nature of the changes which 

 take place on burning substances in air. — The use of phosphorus 

 is introduced by reference to a match. Phosphorus is then 

 burnt under a bell jar over water and the result noted : the 

 disappearance of some of the air again shows that the air is 

 concerned. The fact that phosphorus smokes when taken out 

 of the water in which it is always kept suggests that some 

 change is going on, so a stick of phosphorus is exposed in air as 

 in the previous experiment with iron : soon one-fifth has dis- 

 appeared, and the phosphorus then ceases to smoke. The 



