ON TEACIIINa CHEMISTRY. 237 



have indicated tliat chalk loses something -when burnt — the idea that an 

 invisible something is given off is especially probable after the experi- 

 ments with red lead have been made ; so it is decided to heat chalk 

 strongly, but before doing this chalk and lime are examined comparatively. 

 Chalk is observed not to be altered by water ; on shaking it with distilled 

 water and evaporating some of the filtered liquid in a weighed dish, very 

 little residue is obtained — so it is established that it is but very slightly 

 soluble in water. Lime is slaked, weighed quantities of lime and water 

 beino- used ; the retention of a considerable amount of water, even after 

 exposing the slaked lime in the drying oven, shows that the slaking in- 

 volves a definite change in composition — that slaked lime is lime and 

 water. The solubility of the lime is next determined and found to be con- 

 siderably greater than that of the chalk. It is found that chalk is but 

 very slightly altered in weight when heated over a gas flame, and that it 

 is only when it is strongly heated that it is converted into lime : so th© 

 chalk is strongly heated in an iron tube in a Fletcher blowpipe furnace, 

 ■when gas is freely given off, and subsequently it is found that the chalk 

 has become lime. The gas is tested with a taper, which it extinguishes, 

 so it cannot be oxygen, but may be nitrogen ; its density is therefore 

 compared with that of nitrogen and found to be greater, so evidently ii 

 is a peculiar gas and may be called chalk gas. If chalk consist of this 

 gas and lime, it should be possible to reproduce chalk from them ; so the 

 gas is passed through a small weighed tube containing lime and the tube 

 is found to get heavier. But lime and chalk are so much alike that it is- 

 difficult to say that chalk is formed : perhaps dissolved lime will act simi- 

 larly ; the gas is therefore passed into or shaken up with lime water. 

 The precipitate which forms looks like chalk and probably is, but this 

 remains to be decided. The discovery of this behaviour of chalk gas is- 

 important as affording a means of again comparing the gas from chalk 

 with nitrogen. In working with lime water it is scarcely possible to avoid 

 noticing that a film forms on its surface ; by exposing a quantity of the 

 lime water a considerable amount of the precipitate is obtained : its re- 

 semblance to chalk is noted, and the possible presence of chalk gas in air 

 is thereby suggested ; this and the precipitates previously obtained are 

 collected, dried, and then introduced into pieces of narrow hard glass 

 tnbinf, connected to wash-bottles containing lime water, and on heating 

 strongly by means of a blow-pipe flame, while air is sucked through to 

 carry forward any gas into the lime water, the white precipitates are 

 again obtained, so no doubt remains that the original precipitates were 

 chalk. Incidentally the discovery is thus made that air contains some- 

 thing besides oxygen and nitrogen, viz., chalk gas. 



It being thus established that chalk consists of two things, lime and 

 chalk gas, at this stage it is pointed out how firmly these two con- 

 stituents hold to each other in the chalk. The absorption of the gas by 

 the lime — its entire disappearance in fact — is commented on. Accurate 

 determinations of the loss of weight on heating crystallised chalk (calc 

 spar) should at this stage be carried out before the class, if not by the- 

 pupils, so that the numbers may bo quoted and that it may become 

 impressed on them that the proportions in which the lime and chalk gas- 

 are present is constant. Their attention may be recalled to the oxides 

 previously studied, it being pointed out that on inspection these afford 

 no indication that they contain oxygen : that here again the gas entirely 

 loses its individuality on entering into union or combination. That 

 oxides contain their constituents in fixed proportions may be de- 



