ON TEACniNG CHEMISTRY, 233 



ficial 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 sub- 

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

 tested by heating them with a Fletcher gas blowpipe 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 copper or iron. 



The further study of the effect 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 evaporates and it is then observed that some- 

 thing 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 condensed 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 temperature 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 dissolved 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 car- 

 bonised and the amount of char determined, then burnt and the percentage 

 of ashes determined. A small platinum dish, 15 to 20 grams in weight, is 

 required for these experiments, and a gas muffle furnace is of the greatest 

 use in burning the char and in oxidising metals. In addition to the dis- 

 cipline afforded by such experiments a large amount of valuable informa- 

 tion 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 metals such as lead, copper, tin and iron may be studied ; their 

 increase in weight is in striking contrast to theinalterability of substances 

 like sand and salt, 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 ex- 

 ceptional 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 instance 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 tbrmed 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 



