72 



Mr. E. P. Perman. [Feb. 4, 



"The Evaporation of Water in a Current of Air." By EDGAR 

 PHILIP PERMAN, Assistant Lecturer in the University College 

 of South Wales and Monmouthshire. Communicated by 

 Professor E. H. GRIFFITHS, F.RS. Keceived February 4, 

 Pvead February 19, 1903. 



Introduction and Histwical. 



It may be thought that so simple a matter as the evaporation 

 of water in a current of air has already been thoroughly investigated, 

 but, so far as I have been able to discover, this is not the case, 

 although much concerning it has been taken for granted. 



This research was undertaken in order to ascertain with what 

 accuracy the vapour pressure of water could be calculated from the 

 amount of water vapour carried off by an air current passed through 

 the water, the temperature being maintained constant. 



The work was begun in the Physikalisch-chemisches Institut, 

 Leipzig, during the summer of 1902, and completed at University 

 College, Cardiff, during the same year. 



Experiments of a similar nature have been made by Regnault* and 

 by W. N. Shaw.f Kegnault found that the weight of the vapour 

 drawn off agreed within about 1 per cent, with that calculated 

 from the vapour pressure. The temperatures employed extended 

 up to 45 C. 



Shaw's experiments were at ordinary temperatures of the air, and a 

 very close agreement was found between the usually accepted vapour 

 pressures and those calculated from the amount of water vapour 

 drawn off. 



Description of Apparatus. 



The apparatus employed consisted of the following chief parts : 

 (1) Thermostat, with thermometer and stirring arrangement. (2) 

 Four wash-bottles to contain the water. (3) Absorption apparatus to 

 take up the water-vapour as the moist air passed through it. (4) 

 Aspirator. 



(1) The thermostat (T), consisted of an iron pot 23 cm. in 

 diameter and 21 cm. high, surrounded by thick felt, and heated 

 by an Ostwald burner (0), fig. 1. The thermometer was made by 

 Fuess, of Berlin, and was standardised at the Reichsanstalt, 

 Charlottenburg, number 17,000. It was divided into tenths of 1 C., 

 and was read to 0*01 ; the final corrections were also given to 0'01. 

 At the lower temperatures the stem was allowed to project from 

 the bath just enough to make the thermometer readable, but at 



* ' Mem. de 1'Acad.,' vol. 26, p. 679. 



f ' Phil. Trans.,' A, vol. 179, 1888, Appendix No. Ill, p. 41. 



