80 MR. W. N. SHAW ON IIYGROMETRIC METHODS. 



the Cavendish Laboratory. All these thermometers were, through the kindness of 

 Mr. WHIPPLE, specially compared for me at Kew for every degree Centigrade in 

 January, 1882. Their freezing points have been determined from time to time, and 

 the tables of corrections revised. The temperature readings may be regarded as 

 accurate to within 0'l C. 



4. The apparatus used for the first series of experiments (Table II.), of which fig. 1 

 (Plate 5) shows the general arrangement, consisted of three distinct parts : 



(i.) The aspirator (C) for causing the passage of a known volume of air over the 

 desiccating substances in the drying tubes. 



(ii.) The weighed drying tubes (B, B) for determining by their increase of weight 

 the quantity of moisture in a known volume. 



(iii.) A saturator (A) for supplying saturated air at the temperature of the room. 



Between the drying tubes and the aspirator was placed an additional drying tube, 

 or a bottle filled with chloride of calcium, to prevent moisture reaching the weighed 

 tubes from the aspirator. 



(i.) The aspirator was of the ordinary form, a copper cylinder with conical ends ; a 

 tap was fixed at the bottom, and through a cork at the top passed three glass tubes : 

 the first, for the delivery of the air from the drying tubes, passed nearly to the bottom 

 of the aspirator to ensure a uniform flow of air ; the second, connected with a similar 

 one entering at the bottom, served as a gauge ; the third passed to the lower surface of 

 the cork, and was used to fill the aspirator by being connected with an aspirating 

 pump. The aspirator was also provided with a thermometer, passing through the side 

 of the upper cone. 



The volume of each aspirator was determined by completely filling it with water as 

 for an experiment, and running the water out gradually into a litre flask of known 

 weight when empty, and weighing the flask each time when full, the flask being care- 

 fully dried by means of a hot cloth between successive fillings. 



Two aspirators were employed marked A and B, and the quantity of water in each 

 was determined twice with the following results : 



Aspirator A, 1st observation 163f>5'9 grammes, temperature 18 C. 



2nd 16371-2 



Aspirator B, 1st 16384-0 



2nd 16383-8 



We have therefore, allowing for the density of the water used and the temperature, 



Volume of A at 16384 cc. 

 B at 16400 cc. 



The volumes at any temperature t will then be 



Aspirator A, 16384 (1 + '000052 t) cubic centimetres. 

 B, 16400 (1 + -000052 /) 



