Joly — A Method of Determining the Density of a Gas. 537 



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 volumeno- 

 meter 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 be 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 experiment 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 temperature 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 

 stopcock 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 the volumenometer. 

 All must now be let stand till t ° is attained (half-an-hour will 

 suffice), and the final adjustment of the mercury level to the mark 

 /then effected. By means of a cathetometer the height h, the 

 difference of level between the surfaces at d and/, is read. This 

 will only be some 5 or 6 mms. in general. For this a temperature 

 correction is hardly required ; but a capillary correction, ascertained 

 experimentally once for all, must be applied owing to the different 

 areas of the surfaces in the two tubes. This might be eliminated 



