Constant Pressures and Temperatures. 413 



( Journ. Chem. Soc. 1873, p. 458) suggested the use of mix- 

 tures of sulphuric acid and water of different strengths, which, 

 if the vapour given off is condensed and returned to the mass, 

 boil constantly at certain given temperatures : here the constant 

 temperature is afforded by the liquid and not by the vapour, 

 which is less hot ; hence, if the liquid tends to become super- 

 heated, fluctuations in temperature will infallibly occur. The 

 great objection to this method is that, when a series of tempe- 

 ratures is required (as, for instance, in the comparison of ther- 

 mometers), great inconvenience and loss of time is incurred by 

 having frequently to replace the liquid in the apparatus by 

 another containing a different proportion of sulphuric acid or 

 other substance. 



A series of temperatures can be obtained with one liquid 

 only, if the pressure under which it boils be varied. This 

 method, simple as it appears, is beset with considerable me- 

 chanical difficulties, to surmount which the apparatus described 

 below has been constructed. 



At first sight it would seem that, if the vessel containing the 

 steam be connected with a large closed vessel serving as air- 

 reservoir, all that is necessary is to rarefy or compress the air in 

 the reservoir to the required extent, and to allow the liquid to 

 boil undisturbed. But the steam-bath is necessarily large; and 

 the reservoir therefore must be large also. Now the reservoir 

 must not only be strong enough to stand a vacuum inside it, but 

 must also be capable of supporting an interior pressure of at 

 least 50 lbs. on the square inch ; such a reservoir is not only 

 cumbrous and expensive, but dangerous. Further, any change 

 of temperature in the room alters the pressure inside this reser- 

 voir ; it must therefore be placed in a cistern of water, which is 

 a second objection'. The great obstacle to the employment of 

 this method, however, is the difficulty of preventing leakage 

 even with the greatest possible care. In endeavouring to stop 

 leaks I expended so much time that I gave up the reservoir, 

 and determined to construct an apparatus for maintaining a 

 constant pressure in a given vessel even when it leaked. 



Lothar Meyer (Ann. Chem. Pharm. vol. clxv. p. 303) had 

 already devised an apparatus of this kind, adapted chiefly to 

 fractional distillation under reduced pressure. It consists essen- 

 tially of two vertical tubes, A B and D (fig. 1, PL XIII.), con- 

 nected at the upper part by the lateral tubes E and F; at B an 

 india-rubber tube connects A B with the tube K, which slides 

 up and down the board to which the whole is fixed ; at the top 

 of A B is a tube H which is connected with an air-pump ; the top 

 of C D is provided with a cork, through which a narrow tube S S 

 passes nearly to the bottom of C D ; finally, the lateral tube k is 



Phil. Mag. S. 5. Vol. 7. No. 45. June 1879. 2 I 



