82 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



the weight of the tubes is the weight of the glass, and the specific gravity of the 

 pumice and of the strong sulphuric acid does not differ much from that of glass ; we 

 may, therefore, calculate the correction for weighing in air on the assumption that the 

 specific gravity of the whole tube is that of flint glass, which we may take to be 3 '5. 

 This would make the correction to weight in vacuo for a tube of 150 grammes equal 

 to 30 milligrammes. The effect of a barometric variation of 1 cm. upon such a tube 

 would therefore be to alter its apparent weight by '39 mgm., and a variation of 1 C. 

 of temperature would produce an alteration of '11 mgm. in the apparent weight. 

 The changes in barometric pressure and temperature between two successive weighings 

 may therefore be such as to cause the apparent weight of the tube to alter by a 

 considerable fraction of a milligramme. Now the amount of moisture is determined 

 by the difference of weight of the tube at the two weighings, and accordingly any 

 error in the weighing, due to neglecting to correct for weighing in air, will be of the 

 same absolute magnitude, and of very much greater relative importance, in the weight 

 of moisture absorbed by the tube. The variations of pressure and temperature, 

 however, between the successive weighings of the tubes during the observations were 

 not sufficient to produce any appreciable effect upon the results. 



(iii.) The saturator. This part of the apparatus was similar in principle to that 

 used by REGNAULT. Two long bell-jars stood in a shallow dish of distilled water ; 

 the one jar was filled with well-washed sponge, and the second contained a wire 

 frame covered with muslin, which, with it, stood in the water at the bottom of the 

 dish. The air supplied to the aspirator was drawn by means of a glass tube passing 

 through the cork in the top of the jar from the middle of the muslin cage, and close 

 to the opening of the tube was the bulb of the thermometer, which also passed 

 through the cork ; the place of the air thus removed was supplied by air passing 

 from the outside through the sponge vessel, and delivered into the second vessel 

 outside the muslin cage. During an observation, which lasted generally about two 

 hours, the thermometer was read every quarter of an hour by means of a telescope 

 placed at some distance, in order to avoid any error of parallax, and the mean of the 

 readings taken as the temperature of the saturated air. The muslin cage served to 



amount of moisture in the air of the room, first by pumipe tubes, and secondly by chloride of calcium 

 tubes. The amounts are given below : 



Since the above was written I have found that previous observers have expressed the same opinion 

 about calcium chloride. (See ANDREWS, ' Phil. Mag.,' vol. 4, 1852, p. 330, and MULLEK-EK/DACU, ' Deutsch. 

 Chem. Gesell. Ber.,' vol. 14, p. 1093.) 



