382 Mr. J. Joly on a Method of 



in the copper cylinder, this time in its normal position, and the 

 water allowed to run out. The ice is now returned to the jacket, 

 and after the lapse of some time the volumenometer is closed 

 as before, when the height H of the barometer is read. 

 Another weighing is now made, and by difference the weight 

 of water at 0° C. which has quitted the vessel is calculated, 

 allowing for the weight of saturated air at 0° 0. and H con- 

 tained in the vessel at the second weighing. One or more 

 such operations may be performed, and from the known 

 density of water at 0° C. the volume at the temperature 0° C. 

 calculated. To this must be added, for the complete volume, 

 the volume at ordinary temperatures of the small steel con- 

 nexion with the sphere. It is obvious that the error arising 

 from the inconstant temperature of this fine tube or of the 

 short tubulure exposed at /need not be attended to. 



If mercury is subsequently to be employed in the volume- 

 nometer the volume should be determined on a slightly 

 different procedure. Water should still be used, however — 

 not mercury, or a distending effect, due to the great weight 

 of the latter, might give rise to error — but the weight of the 

 vessel, when containing air, should he first ascertained when 

 the walls within are dry. In this way the capacity of the 

 dry volumenometer is obtained, which is that required with 

 the use of mercury. It is evident that the use of mercury is 

 on this account much preferable to water in subsequent work, 

 as the volume of residual water in the vessel from one experi- 

 ment to another will not be accurately constant. Again, no 

 hygrometric correction on the pressure of the gas contained 

 in the volumenometer will be requisite. 



The volume of this vessel (which is but an enlarged 

 Sprengel tube) being ascertained once for all at the tempera- 

 ture 0° C, it follows that it will afford a ready means of 

 dealing with a gas contained in the weighing-sphere. It is 

 filled with mercury or water, and surrounded by ice. The 

 vessel d is then raised till the liquid in the volumenometer has 

 risen to the top of the fine steel tubulure b, the weighing- 

 sphere removed from the balance, screwed on to b, and the 

 vessel d lowered. The valve at a is next screwed back a little 

 till the mercury is seen to rise in d. The right-hand stop- 

 cock on d is now opened, and the mercury let flow into a tall 

 graduated or marked vessel, so that it may be known about 

 when the operation of filling g is approaching completion. 

 Presently, the surface level of the mercury sinks into the 

 vessel c. This possesses such capacity as to give time to 

 close the valve of the sphere before any gas is lost ; d is now 

 raised slowly, till the overflow gas in c is compressed back into 



