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LXI. On a New Form of Sulphuric- Acid Drying -Vessel. 

 By the Earl o£ Berkeley, F.R.S., and E. G. J. 

 Hartley, B.A* 



IN the course of a prolonged series of measurements of the 

 vapour pressure of aqueous solutions, some of which 

 have already been published t, the method employed has 

 been to saturate a current of dry air with the vapour of the 

 solution and pure solvent respectively. The problem of 

 drying the air-current has thus arisen, and some experi- 

 ments to determine the relative efficiency of phosphorus 

 pentoxide and sulphuric acid as desiccating agents seem to- 

 be worth recording, since comparatively few quantitative 

 results on this subject appear to have been published J. 



Our experiments were designed to throw light on two 

 separate points, and proof was thereby obtained that : — 



(1) Sulphuric acid is capable of drying air as completely 

 as phosphorus pentoxide, at least to the extent that is 

 required in this class of work. 



(2) By emplojdng suitable apparatus it is not necessary to 

 bubble § the air through the acid, but only to lead it over its 

 surface ; thus avoiding both the risk of introducing acid 

 spray into the air-stream and uncertain changes in the volume 

 of the air-current. In vapour-pressure measurements by 

 this method the latter source of error is of considerable 

 importance || . 



In addition to the foregoing it was shown that the vapour 

 pressure of the sulphuric acid itself, to which Mr. J. J Manley 

 has called attention % is too small to be of importance. 



For these experiments a new form of phosphorus-pen t- 

 oxide vessel was devised having some points of advantage 

 over the usual tube ; we give a description of it as it may be 

 of use to others. This vessel is shown in figs. 1 and 2 ; 

 legs are sealed in at the sides at a, thus enabling it to- 

 stand on the balance or elsewhere without support; it is fitted 

 with a ground-on cap b and a ground-on detachable mercury 

 cup c. It may be mentioned here that we have found this 



* Communicated by the Authors. 



t Proc. Rov. Sue. 1*906, A. vol. Ixxvii. p. 156 ; Phil. Trans. A. vol. ccix. 

 p. 177. 



+ Morley, Am. J. Sci. vol. xxx. p. 140, vol. xxxiv. p. 199 ; Shaw. Phil. 

 Trans. A. 1885, vol. clxxix. p. 84. 



§ Moistened pumice is frequently used, hut for accurate work great 

 precautions are required to ohtam suitable pumice and to avoid Bpraj . 



|l ' Nature,' July 1905 ; Proc. Roy. Soc. he. tit. p. 1 65. 



% Phil. Trans. A. vol. ccxii. 1913, p. 243. 



Phil. Mag. S. 6. Vol. 29. No. 172. April 1915. 2 R 



