890 Royal Irish Academy : Dr. Apjohn on the 



Let a known volume of dry air be charged with moisture at any 

 given temperature, and let the expansion produced by the moisture 

 be accurately noted. The pressure being also measured by an ac- 

 curate barometer, we have the means of calculating the force of the 

 vapour which has produced the expansion. For if v be the volume 

 of the dry air, and v' that of same air when charged with moisture, 

 / the force of the vapour, and p the existing atmospheric pressure, 

 we shall have 



from which we deduce 



v' = v x — , 

 P-f 



f=m**- 



It was not my original intention to make any experiments upon 

 the force of aqueous vapour, believing the table which I have hitherto 

 employed, and which was calculated by the author of the article 

 " Hygrometry," in Brewster's Encyclopaedia, from the experiments 

 of Dalton, to have been sufficiently exact. But the correctness of 

 this table having been indirectly called in question by sk> high an 

 authority as M. Kupffer, who has come to the conclusion, that the 

 table of the force of aqueous vapour, given by a German meteorologist 

 of the name of Kamtz, is alone to be relied upon, I resolved to com- 

 mence with the vapour of water, in the hope that I might be able, 

 by the results of direct experiment, to corroborate a conclusion pre- 

 viously drawn by Professor Lloyd, from a discussion of some hygro- 

 metrical observations of mine, viz. that for temperatures within the 

 atmospheric range, the table of Kamtz is less accurate than that of 

 Dalton, the values given in the former being all too low. 



The apparatus I have employed in my experiments is composed of 

 a glass ball prolonged on the one side into a short tube, furnished 

 with a cap and stop-cock, and, on the other, into a long tube of 

 somewhat smaller diameter, divided into 100 equal parts, each being 

 •042 of a cubic inch, or the - 001 of the total capacity of ball and 

 tubes down as far as the division marked 1000. 



The first step consisted in filling this vessel with dry air, which 

 was done in the following manner : into the extremity of the gra- 

 duated tubular portion, a cork pierced by a small tube, open at both 

 ends, was inserted, and this tube was then connected with the orifice 

 of a table air-pump usually occupied by a syphon gauge. The stop- 

 cock was now connected with one end of a long tube, packed with 

 fragments of fused caustic potash, while the other end of this tube 

 was attached by means of a slip of caoutchouc to a second tube 

 passing through an air-tight cork fixed in one of the mouths of the 

 bottle, at present used for the inhalation of chlorine. This bottle 

 being charged with oil of vitriol, and the orifice of the plate of the 

 pump being closed, the pump was worked, and a current of air was 

 thus drawn through the glass vessel for about fifteen minutes, which 

 in passing through the oil of vitriol, and over the fused potash, was 

 deprived of all hygrometric moisture. The included air being now 



