History of Air- Analysis 11 



in knowing the amount of the decrease in volume than the cause. With 

 his keen insight and experimental technique he attacked the problem, and 

 in 1783 published a paper describing a new eudiometer. By carefully 

 noting the rate and the amount of nitric oxide used with this apparatus 

 and particularly by adding the air to a previously measured amount of 

 nitric oxide, he was able to make a more careful examination and test of 

 air. Of particular interest in connection with this paper are his words 

 regarding the tests of air on different days :* 



During the last half of the year 1781 I tried the air of near 60 different days in order 

 to find whether it was sensibly more phlogisticated at one time than another; but found 

 no difference that I could be sure of, though the wind and weather on those days were 

 very various, some of them being very fair and clear, others very wet, and others very 

 foggy. 



He also studied the air in different localities and compared the air of 

 London with that of the country. Although Cavendish found some evi- 

 dence to show that the air at Kensington was better than that in London, 

 he nevertheless believed that the differences were no greater than the 

 limits of experimental error, and taking the mean of all, that there was 

 apparently no difference between them. The number of days compared 

 was 20, the greater part of the samples being taken in cold winter weather, 

 when there were a great many fires and but little wind to blow away the 

 smoke. From figures given in Cavendish's notebook, Wilson 2 concluded 

 that these observations established that the percentage of oxygen in the 

 air was 20.83. This figure has been made much of in discussions of the 

 percentage of oxygen in the air, but an examination of Cavendish's data 

 shows that the limit of error was very large, and that he could not possibly 

 have been inside of 1 or 2 per cent of the total amount of oxygen involved ; 

 hence a representation of his percentage of oxygen with four significant 

 figures is without value. As Cavendish at that time paid no attention to 

 the purity of the nitric oxide, although recognizing in a crude way the dif- 

 ferences in quality and the possibility of combination with varying amounts 

 of dephlogisticated air or oxygen, it is fair to conclude that the method 

 could not possibly have had an accuracy closer than 2 per cent of the total. 

 Cavendish was the first to establish that the composition of the air was 

 essentially constant within the limits of his apparatus, and that it did not 

 sensibly vary in different parts of the country. That it was possible 

 to obtain these results with an instrument and a method with as great 

 error as we now know them to have had is certainly most remarkable evi- 

 dence of his skill as an experimenter. 



Among others who extensively used the nitric-oxide eudiometer must 

 be mentioned Ingen-housz, who, in his researches on plant life and on his 

 travels, used a portable apparatus of his own devising. 3 



1 Cavendish, Philosophical Transactions, 1783, 73, p. 126. 



2 Wilson, loc. cit., p. 41. 



3 Ingen-housz, Vermischte Schriften, 2d ed., Vienna, 1784, 2, p. 242. 



