Henry Cavendish, Joseph Priestley 85 



a number of gases, and he first prepared oxygen by heating 

 oxide of mercury with a burning glass. He obtained hydro- 

 chloric acid by heating spirits of salt, sulphur di-oxide 

 by the action of sulphuric acid on mercury, and ammonia 

 by heating spirits of hartshorn. Cavendish's attention was 

 attracted by an observation of Waltire, who worked with 

 Priestley, that when a mixture of hydrogen and common 

 air was fired, dew appeared on the walls of the glass 

 tubes. This was explained as being a condensation of 

 water which had been present as vapour in the original 

 gases. But Cavendish was able to prove that the water 

 formed was really the result of the combustion of oxygen 

 and hydrogen. In order to interpret correctly the lan- 

 guage in which chemists expressed their results at the 

 time we must remember that oxygen was referred to as 

 " dephlogisticated air," nitrogen as " phlogisticated air," 

 and hydrogen as " phlogiston." Cavendish therefore ex- 

 presses his result by saying " that water consisted of 

 dephlogisticated air united with phlogiston." The conclusion 

 embodies the discovery of the composition of water, which 

 till then was unknown. 



Similar experiments seem to have been made by James 

 Watt, who subsequently claimed priority, but we need not 

 here enter into the discussions to which the dispute gave 

 rise, and which passed without interfering with the subse- 

 quent friendly intercourse between Cavendish and Watt. 



A remarkable research originated in the interest which 

 Cavendish took in the composition of the terrestrial atmo- 

 sphere. By burning various bodies in measured volumes 

 of air, he satisfied himself that the amount of oxygen 

 present was the same in all the samples experimented upon. 

 He noticed, however, that in one of the experiments in 

 which a mixture of hydrogen and oxygen was fired by an 

 electric spark, the resulting water contained nitric acid. 

 This, Cavendish attributed to a remnant of atmospheric 

 nitrogen in the oxygen used, and, following up the matter, 

 showed that nitrogen and oxygen actually did combine 

 under the influence of an electric spark. Absorbing the 

 nitric acid formed, he could observe a shrinkage of volume 

 when sparks were passed through mixtures of nitrogen and 



