Constant Pressures and Temperatures, 417 



the upper fixed point of thermometers ; the vapour, after tra- 

 versing this, passes into the U-shaped condenser 0, from 

 which the condensed liquid runs back into B down the tube 

 E. The end H of this condenser is connected with the lateral 

 tube C of the manometer by means of a very small lead pipe; 

 and thus the liquid can be made to boil under any required 

 pressure. The thermometers or other instruments are placed 

 in the small tubes T T, which are filled with petroleum of high 

 boiling-point. Besides water, the best liquids for generating 

 the vapour appear to be carbon disulphide for low, and purified 

 paraffin oil for high temperatures ; the latter substance I have 

 not hitherto used, as I have had no occasion to make observa- 

 tions at such temperatures. I have, however, made experi- 

 ments with it in a smaller apparatus of similar form, and found 

 that no variation of temperature took place. 



The temperature obtained from these three liquids may be 

 varied from 25° to 300° without unduly increasing the 

 pressure. 



If the temperature in the double tube D D be observed with 

 a thermometer of which each degree is 5 millims. long, no 

 variation can be detected, even with an extremely rigid cathe- 

 tometer, provided, of course, that the barometric pressure does 

 not alter ; the variation due to this cause might, if necessary, 

 be removed by making the cap of the tube F of the mano- 

 meter fit air-tight. When great accuracy is necessary, it is 

 not advisable to decrease the pressure below 100 millims. 



In order to see whether the whole length of D D is at the 

 same temperature, small oblique tubes similar to T were in- 

 serted, one at the top, the other at the bottom; a thermometer 

 placed first in one and then in the other of these, gave exactly 

 the same reading in both. 



To sum up. First, with the above apparatus, viz. a mano- 

 meter communicating by means of a specially constructed tap 

 worked by a double electromagnetic clutch with a constantly 

 working air-pump, any given pressure may be maintained for 

 an indefinite period without varying more than 0*25 millim.; 

 secondly, if this constant pressure be applied to a suitable 

 vapour-bath, any given temperature between 25° and 300° 

 may be maintained absolutely constant as long as no chemical 

 change occurs in the liquid whence the vapour is derived. 



