MB. W. N. SHAW ON HYGROMETRIC METHODS. 149 



took up 0767 gramme of moisture, being precisely the same amount as that lost by the sponge vessel. 

 My attention has lately been called to the papers by E. W. MOBLKT (' Amer. Journ. Sci.,' vol. 30, 1884, 

 p. 140 ; vol. 34, 1887, p. 199), in which the question of residual moisture in air passed over sulphuric 

 acid or phosphoric anhydride is more rigorously treated. It appears that the residual moisture would 

 not appreciably affect the weighings in the observations here recorded, for sulphuric acid only leaves 

 nnabsorbed " not far from a fourth of a milligramme of moisture in 100 litres of a gas," and " the 

 moisture left unabsorbed by phosphorus pentoxido, if capable of determination, may be very roughly 

 stated as possibly a fourth of a milligramme in 10,000 litres." 



NOTE, APBIL 23, 1888 (see p. 97). 



The researches of WABBURO and IHJJORI (' Wiedemann, Annalen,' vol. 27, 1886, p. 481) afford some 

 information about the formation of a deposit. On a specimen of fresh lead glass at about 16" G. 

 exposed to the vapour of water in vacua a deposit of 18 X 10 * grammes per square centimetre was 

 observed when the vapour pressure inside the vessel corresponded to the temperature 4'71 C. ; the 

 deposit increased to 194 X 10~ 8 grammes per square centimetre when the vapour pressure was increased so 

 as to correspond to a temperature 0'87 C. below that of the glass surface. No results are given for lead 

 glass which enable one to form an opinion as to what happens when the difference of temperature is 

 made very small, but for Thuringian glass a sudden increase of 50 per cent, in the deposit is shown for 

 a diminution of the temperature-difference from 0'27 C. to 0'17 C. With lead glass, of which even a 

 fresh surface showed comparatively very feeble absorption of vapour, no absorption at all waa detected 

 after treatment with boiling water. The apparatus referred to in the text was, excepting the three-way 

 tap, constructed of lead glass that had been frequently washed with cold water. 



