OXYGEN AND OZONE. 51 



periment by which Dr. Priestley, an English chemist, made 

 the discovery of this gas a little more than a hundred years 

 ago, on the 1st of August, 1774. It was discovered also by 

 Scheele, a Swedish chemist, shortly after, he not having 

 heard of the discovery by Priestley. The gas was called 

 by Priestley dephlogisticated air, for reasons which we will 

 explain to you. Very crude and fanciful notions prevailed 

 at that time, and among others that of Stahl, a German 

 chemist, who maintained that all combustible substances 

 burn in consequence of an element in them which he called 

 phlogiston. Now as this gas, while it makes other things 

 burn brightly, does not burn itself, Priestley considered it 

 as destitute of phlogiston, or dephlogisticated. Some years 

 after, the investigation of the qualities of this gas having 

 in the mean time been diligently prosecuted, Lavoisier, a 

 French chemist, gave it the name which it has retained to 

 this day, and which it probably always will retain viz., 

 oxygen. It is derived from two Greek words, oxus, acid, 

 and gennao, I give rise to. His idea was that this gas 

 is a component of all acids. This has since been found 

 not to be true ; but the name is nevertheless retained. 



49. Another Mode of Obtaining Oxygen. A more easy 

 and convenient way of obtaining oxygen from oxide of mer- 

 cury than that described in 47 is represented in Fig. 4 

 (p. 52). Here the oxide is put into a retort, a, where it is 

 heated by the flame of a spirit-lamp. You see also a re- 

 ceiver, b, from which a bent tube, c, passes under the water 

 in the pneumatic trough, g, where its end is directly under 

 the open mouth of a glass jar. The heat of the lamp de- 

 composes the oxide, so that in place of this compound sub- 

 stance we have two elements, mercury and oxygen. But 

 the mercury, as it separates from the oxygen, is, on account 

 of the heat, in the form of vapor, and therefore passes on 

 with the oxygen gas through the tube of the retort. By 



