NITRE, NITRIC ACID, AND NITROGEN 



t8 9 



necessary for the combination of azote with oxygen can be 

 obtained without the addition of inflammable air or of any 

 other foreign substance by using electric sparks. Priestley 

 had noticed that these diminished the volume of air and 

 thought that fixed air was produced. Cavendish confirmed 

 the diminution of volume, but found that the production of 

 fixed air was due "to the burning of some inflammable 

 matter in the apparatus," and "that the real cause of the 



FIG. 39. CAVENDISH'S APPARATUS FOR SPARKING AIR 

 OVER MERCURY. 



diminution " was the conversion of azote into nitric acid. 

 Cavendish writes : 



" The apparatus used in making the experiments was as 

 follows. The air through which the spark was intended to 

 be passed was confined in a glass tube M, bent to an angle, 

 [as in Fig. 39], which, after being filled with quicksilver, was 

 inverted into two glasses of the same fluid, as in the figure. 



By means of a small glass tube suitably bent : 



" I was enabled to introduce the exact quantity I pleased 

 of any kind of air into the tube M "; and by the same means, 



