120 ELEMENTS OF LABORATORY 



That a colourless gas escapes during the operation may be 

 proved by showing that a flame becomes much brighter when 

 held over the heated substance. It is better to try this with 

 another portion of the substance in a test-tube. The gas is 

 called oxygen. 



The process should now be repeated with a different 

 quantity of silver iodate. It will be found that the quantity 

 remaining bears the same ratio in each case to the quantity 

 taken. This will be found to hold good however many times 

 the experiment is performed. Or we may say, that the rela- 

 tive quantity of oxygen contained in the substance is fixed. 



The experiment itself gives no indication of the relation 

 existing between the solid left in the crucible and the gas 

 which has escaped. It remains to be shown that they are 

 associated in a special manner, for which the term ' chemical 

 combination ' is used. The process described is spoken of as 

 one of c chemical decomposition.' It will afterwards be shown 

 jtiow such an association of different kinds of matter differs 

 from a mere mixture or a solution. 



We are chiefly concerned at present in noting that some 

 substances cannot be raised in temperature beyond a certain 

 limit without undergoing decomposition. It may be observed 

 that the decomposition of silver iodate takes place at a lower 

 temperature than that of silver nitrate ] but in each case the 

 temperatures are too high for direct measurement with a 

 thermometer. We may describe these facts by saying that 

 these two kinds of matter do not exist as such above certain 

 temperatures. The same may be said of many other kinds of 

 matter. 



79, Modification produced when Silver Nitrate is heated 

 within a Closed Tube. This experiment should not be per- 

 formed by students themselves, but the results are of sufficient 

 importance to require that it should be shown to them. 



A piece of thick hard glass tubing is drawn out by heating 

 in the blow-pipe flame so as to close one end. A small 

 quantity of silver nitrate is introduced, and the tube drawn 

 out again so as to form a closed tube of three or four inches 

 containing the silver nitrate (fig. 41). The glass should be 



