Joly — A Method of Determining the Density of a Gas. 535 



which the gas is weighed is small, the gas being under high pres- 

 sure within it. That in which the volume is subsequently measured 

 may, on the other hand, be as large as may be desired. We are 

 not concerned with its weight. 



The vessel which I have in use for holding the gas, when being 

 weighed is of copper, spherical in form, about 6*7 cms. in diameter, 

 made up of two hemispheres, the walls being about 1mm. in thick- 

 ness. The hemispheres are brazed (not soft-soldered) together 

 within a belt of copper about 4£ mm. wide, and 1 mm. in thickness. 

 The vessel must be quite staunch. Its internal volume is just 160 

 c.cs. It is closed by a small screw valve, having a side tubulure 

 for attachment to the various apparatus. The valve spindle of 

 steel works through a small stuffing-box to prevent the escape of 

 gas along the shaft of the spindle when the vessel is being filled or 

 emptied. 



This vessel is tested hydraulically to 1000 lbs. in the square 

 inch. It may be safely filled to pressures of 20 or 25 atmospheres. 

 I am daily using such vessels in the steam calorimeter in deter- 

 mining the specific heats of gases; here they are raised to the 

 temperature of 100°O. bringing the pressure to near 30 atmospheres 

 in many cases. For the purpose of compressing the gas into this 

 vessel I use a pump of special construction, a description of which 

 I hope shortly to publish. But, obviously, any compression pump 

 which will deliver the gas without contamination of oil or moisture 

 will answer. 



A counterpoising sphere of the same bulk and weight approxi- 

 mately is required. Against this, after the required quantity of 

 gas has been inserted in the weighing sphere, the latter is 

 equilibrated. This operation is readily performed in an ordinary 

 chemical balance to an accuracy of one-tenth or one-twentieth of 

 a milligram. The weight of a copper vessel of the strength 

 described will be from 80 to 90 grammes. It will be seen later 

 that one sufficiently strong to hold 2 litres safely need not exceed 

 some 40 grammes in weight. 



The vessel in which the volume is determined is next to be 

 considered. It is lettered g, in Plate XL, where it is shown 

 connected with the weighing-sphere, the latter screwed to the steel 

 connexion b, provided for it, and through the fine Jbore (1 mm.) of 

 which it delivers its contents into g. The volumenometer is of 



