46o Description of a Manometer. 



peraturc be different, we ought to bring back the gas to the 

 first by calculation. 



This calculation requires that we should take in not only 

 the change of elasticity produced by the temperature, but 

 also that which flows from the tension of the vapour of the 

 water which is formed or destroyed; and for the latter pur- 

 pose we may use the observations of Mr. Dalton. 



After having ascertained the variations which have taken 

 place in the elasticity at different times of the observation, 

 it is important to be able to determine the chemical changes 

 which have taken place in the atmosphere of the vegetable 

 or animal substance, and the nature of the gaseous sub- 

 stances which may be liberated or absorbed. 



This object is attained by means of a stop-cock, above 

 which we adapt into a reservoir a graduated tube filled 

 with water : on opening the stop-cock the water falls into 

 the manometer, and its place is supplied in the tube by an 

 equal volume of gas : the stop-cock is closed, and we may 

 carry the tube with the gas which it contains. 



We thus obtain a quantity of the gas contained in the 

 apparatus, every time that we wish to examine it, without 

 producing any change in the pressure of that which remains, 

 and in the elevation of the barometer : it is only requisite 

 to submit the gas which has been extracted to chemical 

 tests. 



We determine the proportion of carbonic acid by the 

 absorption of lime water, afterwards that of the oxygen by 

 the hydrogenated sulphuret of lime, according to the 

 method of M. de Marty* : and lastly, we test the residuum 

 with oxygenated gas in the eudiometer of Volta, if we 

 suppose an inflammable gas to exist in it. The remainder 

 gives the proportion oi' the azote. 



In most circumstances carbonic acid is formed, and 

 more or less of it is dissolved in the water which has been 

 introduced into the apparatus, according to its quantity, 

 temperature, and the pressure to which it is submitted. 

 M. Theodore de Saussure, in order to determine the quan- 

 tity of carbonic acid which was absorbed in several of his 

 experiments, contented himself with regarding it as equal 

 to the volume of water which was in his apparatus. This 

 determination is not sufficiently rigorous, since the quantity 

 which is absorbed by the water varies much by the circum- 

 stances which have been detailed. 



The quantity of carbonic acid which has been absorbed 



* Journal de Physique, tome lii. dnnalts de Chimie, tome Ixi. 



by 



