ATMOSPHERE. 



53 



in tlie application of various substances to a given 

 bulk of air, which have the property of absorbing 

 and removing the oxygen, but which do not act upon 

 the azote. The diminution of bulk gives the quan- 

 tity of oxygen ; the residue that of azote. The ap- 

 paratus contrived for these experiments, received the 

 name of eudiometers, because they were considered 

 at first as measurers of the goodness of air. For it 

 was supposed that the proportion of oxygen was va- 

 riable, and that the salubrious or noxious qualities of 

 the air depended upon that proportion. Ingenhousz 

 thought he discovered, that the atmosphere above the 

 sea contained more oxygen than that above the land ; 

 hence he accounted for the supposed salubrity of 

 the sea air, which has been highly extolled from the 

 remotest times. 

 Nitrous pas The first eudiometer was applied, in consequence 

 eudiouie- of Dr Priestley's discovery, that nitrous gas absorbs 

 ter. the oxygen from common a.r. When nitrous gas 



comes in contact with oxygen gas, they immediately 

 combine and form nitric acid; and if the mixture be 

 standing over water, the acid is immediately absorbed 

 by the liquid. Hence the bulk of a mixture of ni- 

 trous gas and common air immediately diminishes, 

 and the diminution is proportional to the quantity of 

 oxygen in the air, supposing all other circumstances 

 the same, and of course measures that quantity. Dr 

 Priestley's method was, to It up into a gradua- 

 ted tube 100 measures of air, and then to add 100 

 measures of nitrous gas. The mixture became yel- 

 low, and its bulk diminished. He denoted the good- 

 ness of the air by the residual gas. Thus if 114 

 parts remained, he said that the goodness of the air, 

 by the test of nitrous gas, was 114; of course the 

 smaller the residue, the greater was the goodness of 

 the air. This method did not ascertain the absolute 

 quantity of oxygen. It was soon observed, that even 

 when the air operated upon was absolutely the same, 

 the residue was liable to considerable variation from 

 apparently trifling circumstances. Thus, for example, 

 it the tube was agitated during the mixture, it was 

 observed that the residue was always much less than 

 if no agitation was applied. If the tube was narrow, 

 the residue was always more considerable than if the 

 tube was wide. The purity of the water, too, over 

 which the experiment was made, had considerable 

 influence. Mr Cavendish observed, that if the water, 

 was in such a state, that it frothed when agitated as 

 if it had contained soap, then the residue was always 

 less than it otiierwise would be. 

 Improved. T ne a PP aratlls was much improved by Fontana, 

 who regulated the size of the tube and the manner 

 ' of mixing the gases ; hence the instrument is usually 

 known by the name of Fontana's eudiometer. This 

 eudiometer was employed by Ingenhousz, and the 

 variations which he found in the compositions of the 

 atmosphere, were obviously owing to the errors to 

 v. hich it was liable. Mr Cavendish first pointed out 

 the precautions necessary, in order to ensure accuracy 

 when this eudiometer was employed. But before 

 nitrous gas couid be used with advantage in the ana- 

 lysis of air, it isary to ascertain the propor- 

 tion of it which combined with a given bulk of oxy- 

 gen gas. r I not undertaken by Mr Dal- 

 ton, {Phil. Mag. xxiii. 351.) According to him, 



21 parts of oxygen gas are capable of uniting either 

 with 36 measures of nitrous gas, or with 2 X 36=72 

 measures. Both of these compounds are soluble in 

 water. If the tube in which the mixture is made be 

 wide, and if agitation be employed, the two gases 

 come at once in contact, so that the oxygen com- 

 bines with a maximum of nitrous gas. If the tube 

 be narrow, and if no agitation be employed, the oxy- 

 gen gas combines with a minimum of nitrous gas. 

 In tubes of intermediate bore, the proportion of ni- 

 trous gas which combines with the oxygen, is inter- 

 mediate between 56 and 72. Hence his rule is to 

 employ a tube of so small a bore, that water can just 

 be poured easily out of it ; to put up into this tube 

 the quantity of air to be examined, and then to let 

 up a quantity of nitrous gas equal to about half the 

 buik of the air ; to allow this mixture to remain two 

 or three minutes without any agitation, and then to 

 observe the diminution of bulk. This diminution is 

 to be multiplied by fi, or 0.368. The product is the 

 bulk of the oxygen gas contained in the air examin- 

 ed. Suppose we employ 100 measures of air, and 

 let up 50 measures of nitrous gas, and that the dimi- 

 nution of bulk amounts to 57; then 57x0.368= 

 20.976, or very nearly 21. This indicates, that the 

 100 measures of air contain 21 measures of oxygen, 

 gas. We have tried the method of Dalton very fre- 

 quently, and have found that when the tube is suf- 

 ficiently narrow, and the experiment carefully made, 

 the mean error cannot be rated higher than 1 per cent. 

 When the gas examined contains much more oxygen 

 than common air, and above all, when it is almost 

 pure oxygen, the -error is greater ; so great, indeed, 

 that the method cannot be depended on. 



But though Dalton's method is correct, as far as 

 the analysis of atmospherical air is concerned, there 

 can be little doubt that the proportions which he has 

 assigned as the limits in which oxygen gas and nitrous 

 gas combine are incorrect. We have made many 

 triah to ascertain these limits, but never could ob- 

 tain the proportions given by Mr Dalton. Mr Gay 

 Lussac has lately turned his attention to this subject, 

 and has given a very simple and satisfactory account 

 of the proportions in which the two gases combine, 

 (Memoires 11' Arcueil, torn. ii. 233.) In a paper 

 which he published on the combination of gaseous 

 bodies, he shewed, that in all cases they unite ei- 

 ther in equal bulks, or one part in bulk of one with 

 two or three parts of another, and in no intermediate 

 proportions. This opinion was obviously founded on 

 a very ingenious hypothesis of Mr Dalton, relative 

 to the way in which substances combine. This led 

 him to examine the combination of oxygen gas and 

 nitrous gas. The result was, that 100 parts of oxy- 

 gen gas unite either with 200 or with 300 parts of 

 nitrous gas. The first compound constitutes nitric 

 acid, the second nitrous acid. His method of ana- 

 lysing air, founded upon this discovery, is to let up 

 100 measures of air into a wide vessel, and then to 

 add 100 measures of nitrous gas. In about a minute 

 the absorption is completed. No agitation is to be 

 employed. The fourth part of the diminution gives 

 the oxygen. Suppose the diminution to amount to 

 84, the fourth of that number, or 21, represents the 

 oxygen in 100 parts of air. On repeating this expe- 



Atmo- 

 sphere. 



