CHAPTER III. 



OXYGEN GAS MAKING. 



[HE first requisite for the lime-light is an adequate 

 quantity of oxygen gas, and this chapter will, 

 therefore, be devoted to the details of its manu- 

 facture, or rather its separation from those substances with 

 which, in nature, it is associated. For, although the most 

 abundant of all the elements, oxygen does not occur in 

 the uncombined state, and, therefore, the chemist has to 

 be at the pains of separating it from its various yoke- 

 fellows. There are several methods of obtaining this gas ; 

 the greater number of which, being only of experimental 

 interest, may be passed over. 



The gas was originally discovered by Priestley in 1774, 

 and at about the same time, independently, by the Swedish 

 chemist, Scheele. Priestley obtained it by heating mer- 

 curic oxide in a flask, which substance, under such treat- 

 ment, breaks up into mercury vapour and oxygen gas. 

 Such a method is clearly out of the question, when several 

 feet of gas are required for the lime-light. 



Another method, which is applicable when large quan- 

 tities of oxygen are wanted, and which has long been 



