COMPARISON OF PLATINTM ANI> <:.\S THKK'MoMKTKI 73 



returns to the boiler by a tube plunging below the water-level. All the communi- 

 cation tubes between the different parts are wide, and arranged so as to avoid the 

 possibility of their becoming choked by the condensation of water in them. The 

 excess of the interior pressure over that outside can be measured by a small water- 

 manometer introduced into the cork. 



XXIX. COMPARISON-BATH FOR RANGE 80 TO 200. 



The disposition of this apparatus, indicated in fig. 17, is the result of a long series 

 of experiments, the aim of which was to obtain a bath sufficiently uniform in 

 temperature to be employed for the accurate comparison of mercury thermometers 

 with the gas thermometer over the range mentioned. It fulfils satisfactorily the 

 principvl requirements of an apparatus of this kind, viz. : 



(1) Uniformity of temperature throughout a space of large dimensions. 



(2) Rapid re-establishment of a steady state after the pressure in the boiler hsis 

 teen altered. 



(3) Employment of a small number of inexpensive liquids easily obtained in a state 

 of sufficient purity. 



The lx>iler consists of a cylindrical vessel of planished copper 2 millims. in thickness ; 

 it has a diameter of 17 centims. and a height of 82 centims. A bell-shaped vessel of 

 the same material is soldered by its rim concentrically into the interior of the cylinder. 

 This inner vessel is filled with a heavy petroleum oil, in which the reservoirs of the 

 thermometers to lie compared are directly immersed. 



The stirring is effected by a vertical stirrer (not shown in the figure), the stems ol 

 which emerging from the loath are protected against cooling by glass tubes. The 

 annular space between the two vessels serves for the circulation of the vapour, and to 

 increase the uniformity of this circulation the space is divided into two approximately 

 equal parts by the introduction of a thin tube of copper open at both ends, and 

 resting on the bottom of the cylinder. The vapour given off by the boiling liquid, 

 which fills the lower part of the outer vessel, rises first in the interior space in contact 

 with the walls of the oil-bath, then descends by the exterior, again ascending into 

 the condenser placed at one side, whence it passes in the state of liquid back to the 

 boiler by a lateral tube. The reversed condenser is in communication by a wide 

 tube with a large copper reservoir in which the pressure can be varied at will, or kept 

 constant, thus changing the temperature of ebullition by a considerable amount ; by 

 using only three liquids any temperature between 80 and 200 can be quickly 

 attained and kept extremely constant for any length of time, provided only that the 

 joints in the whole apparatus remain perfectly tight. A mercury manometer indicates 

 the pressure of the vapour. The bath is covered with several layers ot asbestos-card 

 to avoid losses by radiation and their effects on the temperature of the room. 



VOL. cxciv. A. L 



