( 238 ) 



as in Coiiini. N". 83, and llic thci-monioter / sealed on Ic» in^ while 

 the vapour of the licpiid gas in c\ is exliausted through m.^. 



On the tube c at (\ the glass cap /', is sealed on, through which 

 with an india-rubber stopper the capilhirv connected to c passes at 

 /\. This cap forms the covei- of the larger cryostat. Through this the 

 apparatus has l)econie vei-y tirni and very easj to handle. The 

 capillary b^ is [)i'otected against the stirrer by a metal trame ii. 



Through the cover at /', pass the wires by which the stirrer is 

 suspended. At /'^ the vapour of the bath of liquetied gas is exhausted. 



The surfaces of d and (\ are made retlectors by platinizing them 

 at a redhot temperature with [)latinum chloride in camomileoil. 

 Though the platinum surface is not so shining as that of a silver 

 mirror, yet the advantages of i)latinum for this purpose are evident. 



The regulalioji of the temperature in the two cryostats of the appa- 

 ratus is [)erformed by means of the same exhaust-pump that keeps 

 the pressure constant by means of a pressure regulator (according 

 to the principle of Comm. N" 87, § 3, March '04) in a main tube 

 whicli branches off into two exhaust-tubes, eacii shut with a cock. 



By adjusting these cocks we can take care that the teni[>erature 

 in the inner ciTOstat is a little below that of tiie outer cryostat. What 

 ditference of temperature can he ke|)t constant in the two depends 

 on the temperature itself and on tiie hcpiid gas. 



In measurements to be described in the continuation of this paper 

 the temjierature of the large cryostat was — 25°.() C, that of the 

 small one — 25°. 1 C. The required decrease of pressure in the main 

 exhaust-tube could be easily kept u|) v'>oiling-point methylchloride 

 — 23° C.) with a water air|)iiuip. 



The pressure regulator was adjusted at about the [)ressure belonging 

 to — 25° C. By means (jf the i-eguhiting cocks aji assistant took 

 care, as signs were given l)y another assistant according to the 

 thermometer readings, that l>oth temperatures, hence also the ditfe- 

 rence between them, remained constant. This could t)e attained to 

 within 0°.05 C. 



The accuracy whicli we can reach in the determination of the 

 dew-point with our ap[)aratus depends in the lirst place on the 

 difference of temperature in the two cryostats. As a matter of course 

 it IS smaller as the temperature coefHicient of the begin condensation 

 pressure is larger. At a given ditference of temperature it increases 

 according to the ditference of the pressures at which the conden- 

 sation appears or disappears. The amount of this ditference is also 

 determined by the illumination and this is much impaired by the two 

 walls of the cryostat. In our experiments, observations made with the 



