32 liKNSITIES ()|- <)XY(!KN AND UYDKOCIKN 



If we increase the mean l)y oiie thirtv-tlioiisaiulth,* we get 



1) = 1. 42875) gr. + 0.000();/.4. 



20, SECOND iMKTllOD (H" J)KTKUMlNlNO DENSITY. 



lu the second series of deteniiination.s of the weight of oue litre of oxygen, the 

 barometer and thermometer were not used to measure the temperature and pressure 

 of the gas at each observation. A glolje like those used for the weighings was con- 

 nected to a delicate differential manometer and tilled with pure dry hydrogen. It 

 was surrounded with melting ice, the open branch of the differential manometer wa^ 

 connected to the syphon l)arometer mentioned before, and the pressure I'equired 

 to bring the differential inaiionieter to eiiuilibrium was elaborately determined. 



\\ lien afterwards a globe was tilled with oxygen, it was connectetl with the 

 open branch of the dift'ereutial iiiaiiometer, the temperature of the two globes was 

 made the same, and the pressure of the oxygen was made such as to produce 

 equilibrium in the manometer. It is obvious, assuming for a moment that oxygen 

 and liydrogen have the .same coefficient of expansion, that if the gases in the two 

 globes had the same pressure at some unknown but uniform temperature, the}' would 

 also have the same pressure at the temperature of melting ice, which pi-essure has 

 been detei-mined for the hydrogen. For the actual difference in the coefficients 

 of expansion we can make a nuraeiical correction. 



One advantage of tlie method is that we may observe the equilil)iium of [ires- 



snre and temperature as long as we please, wliere;is, when we measure pressui'e and 



temperature, we are limited to the reading at the instant of closing the globe. 



This advantaije can also be obtained bv siirioiindin<; the srlobe with meltinij ice. 



A second advantage is that the globi- in which the gas was weighed by this method 



was ke[)t untouched, contained in a ilesiccator and suriiMimled by (li\ air, during 



the two detcrinin.'ition^ of the weight of the globe empty and of the glolie tille<l 



with oxygen. 



* See note, page 28. 



