6 Operations used in Quantitative Analysis [7 



by heating for a longer or shorter time at ioo° C, but com- 

 bined water often requires a very much higher temperature for 

 its removal. 



7. The apparatus used for keeping substances at a con- 

 stant temperature of ioo° C, or thereabouts, is shown in fig. 5. 

 It is known as a steam oven, and consists of a copper oven with 

 a double coating. The interval between the two coatings is 



Fir,. 5. — Steam Oven. 



partly filled with water. At one side is an arrangement for 

 keeping the water at a constant level in the oven. 



Exercise II. — Place the watch glass containing the oxalic 

 acid which has been weighed out in Exercise I. in the steam oven, 

 leaving the clamp in the desiccator. After it has been heated for 

 two hours, replace the clamp and allow the whole to cool in the 

 desiccator ; then weigh it. Again remove the clamp and restore 

 to the oven for half-an-hour, allow to cool in the desiccator, and 

 weigh. Repeat this process until two consecutive weighings do 

 not differ by more than one milligram. It may now be presumed 

 that all the moisture which can be driven off at ioo° C. has gone. 



