APPENDIX 177 



clean in use and requiring little attention. Some of the dry cells 

 are much to be recommended on the score of cleanliness. The bichro- 

 mate cell is useful, but tiie cover containing the zinc and carbons 

 is not generally made solidly enough. The Daniell cell is a most useful 

 form for constant circuits. If Grove cells are used, they should be 

 placed outside the room, as the fumes are noxious. A flat window- 

 sill serves very well for the purpose. Great saving of expense in 

 cells may be effected by buying sheet zinc and copper from wholesale 

 houses, and cutting into required shapes. Porous and glazed pots 

 may also be bought directly from the makers. 



7. Observations of Solution, &c. For dissolving solids, beakers 

 or boiling-tubes may be used. These should be made of evenly thin 

 glass. Glass vessels which vary in thickness are very liable to crack. 

 For evaporation, a large free surface of liquid is most suitable ; thin 

 porcelain evaporating basins should therefore be used. The larger 

 the diameter the more regular is the evaporation. A copper water- 

 bath is most suitable for slow evaporation, but when the amount of 

 evaporation required is large, it should be commenced more rapidly 

 by more direct Leafing, as with a sand-bath, and completed on the 

 water-bath. A large water-bath, with openings for a dozen basins, 

 is most economical and least troublesome for class-work. There is 

 however, always a disadvantage in an operation not being carried 

 on, as completely as possible, under the direct control of the small 

 group of boys engaged in it. An operation which is going on in 

 the same manner for all is apt to lose its individuality, and the 

 interest in it is likely to suffer as soon as the feeling of proprietorship 

 relaxes. Competition for accuracy is sure to result from several 

 groups working out, quite independently, the same experiment. 



8. Purity of Substances. An important part of the training of a 

 beginner in chemistry is to learn when a substance may be described 

 as pure when it really contains what its name denotes. Several 

 tests have been described in the previous pages. It must always be 

 remembered that absolute purity is only attained occasionally and 

 with great difficulty, and it is much better that the final stages of 

 purification should be carried on in the laboratory, with the special 

 object of purification in view. It is not reasonable to expect manu- 

 facturers to provide pure materials except at very high prices, and it 

 is unwise to lose the opportunities of observation which such opera- 

 tions afford. Nothing can be more misleading than those descriptions 

 of chemical changes which omit to state that the reacting substances 

 are not pure, and which convey the impression that chemical changes 

 are simple enough to be amply described by an equation. They are 



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