Specific Gravity of Vapours and Gases. 7 



Among the glass vessels there is always one (in this ease the 

 vessel IV.) which contains no glass threads. This one serves 

 for the reception of the gas or vapour whose specific gravity is to 

 be determined. 



The vessel which contains the fewest glass threads (in this case 

 III.) serves for the reception of dry atmospheric air, with which 

 the gas or vapour is to be compared. The vessel V., which con- 

 tains most glass threads, is completely freed from air by means of 

 a mercurial air-pump and then sealed up. The vessel II., con- 

 taining air in any condition is sealed up, and serves in all cases 

 as the tare on the right pan of the balance; but it is still more 

 convenient to exhaust this vessel also and seal it up, in order to 

 make all the weighings on one and the same balance-pan. 

 Lastly, the vessel I, is kept in reserve, to be ready if one of the 

 others should break or become useless. Let us suppose that IV. 

 contains the gas or vapour to be investigated, that III. is filled 

 with dry atmospheric air, and that both are closed with the 

 caoutchouc connectors when at one and the same unknown 

 pressure and at one and the same unknown temperature ; then 

 the difference of weight between the vacuous vessel V. and the 

 vessel IV. filled with gas or vapour, divided by the difference of 

 weight between the same empty vessel V. and the vessel III. 

 filled with dry air, is the specific gravity sought. 



It is convenient to make all weighings according to one and 

 the same method, in order to avoid confusion and errors in cal- 

 culation. Each single weighing consists of three elements : — 



(1) The use of the weight p, for weighing to a centi- 

 gramme. - 



(2) The use of the milligramme rider on the divided scale m, 

 for weighing to a milligramme. 



(3) The observations of oscillations Sr, for weighing to a tenth 

 of a milligramme. 



Suppose, whilst II*. and II. are weighed as tare on the right pan, 

 and IY 7c and IV., containing the gases or vapours weighing Py, 

 are laid on the left pan, that the values of the three elements are 

 p t , nig, dr g , in which, when the weights are laid on the right pan, the 

 value of p t is negative, when they are laid on the left pan it is 

 positive. When m is placed on the right arm of the beam it is 

 positive, when on the left it is negative, and the deviations of 

 the vertical pointer are to be considered positive towards the 

 left and negative towards the right. The weight G calculated 

 for the zero of the scale is then 



G = T v + A IV + T 5 -f A, +& + m g + $r g + P,. 



If I V k and IV. be now replaced by the vacuous vessel V. and 

 Y k , and the three new elements p gi m x , and h\ again deter- 

 mined for the weight G equal to the former one, we have the 



