120 Ml{. \V. N*. SHAW <)N' HVr.KOMKTHir METHODS. 



modified since that time, so that the observations require to be re-calculated before 

 any opinion can be formed about them. 



An apparently complete enumeration of hygrometrical methods is given in the article 

 "Hygrometer" in GEHLER'S ' Worterbuch' (1829), already alluded to. Upwards of 

 50 different instruments and modifications are described. A section is devoted to the 

 theory of the different methods, which it is needless to reproduce. The author prefers 

 the psychrometer to DANIELL'S hygrometer, and is of opinion that it is impossible to 

 construct a truly scientific instrument founded on the alteration of dimensions of animal 

 or vegetable substances. 



I pass on now to the consideration of the work bearing upon hygrometric measure- 

 ments by REGNAULT. That which was directly intended for the elucidation of the 

 subject is contained in two papers published in the ' Annales de Chimie,' and entitled 

 " Etudes sur 1'hygrome'trie."* 



The first part of the first memoir is devoted to the discussion of the fundamental 

 data of all hygrometric calculation, viz. : 



(l.) The saturation pressure of aqueous vapour in the air at different atmospheric 

 temperatures. 



(2 ) The specific gravity of aqueous vapour in saturated air referred to dry air, at the 

 same temperature and pressure. 



(3.) The specific gravity of aqueous vapour referred to air, at the same temperature 

 and pressure, when the fraction of saturation is less than unity. 



The discussion of the first question amounts to an investigation of the truth of 

 DALTON'S hypothesis with regard to saturated air, namely, that the saturation pressure 

 of water vapour in air is the same as it would be at the same temperature in vacuo ; 

 so that in any hygrometric calculation the saturation pressure of vapour may be taken 

 from the table of pressures of water vapour in vacuo for different temperatures. 



For the experiments we are now referring to, REGNAULT used a slightly modified 

 arrangement of his apparatus for determining the vacuum pressures, and obtained the 

 saturation pressure in air and nitrogen gas. The pressure in air was observed for 34 

 temperatures lying between the limits C. and 38 C. The results obtained show a 

 pressure in air less than that given by the table for vacuum, by amounts varying 

 between '10 mm. and 74 mm., the mean of the differences between the two tables of 

 observations being '44 mm. 



The results obtained with nitrogen gas are very similar, the mean differences for 

 the first of two sets of observations being '56 mm. The differences, though very 

 irregular in amount, are considerable, and are always in the same direction, and might, 

 therefore, be held to show that DALTON'S law is only approximately true. REGNAULT, 

 however, suggests that they may be due to some constant error which he could not 

 discover, and later, in the same memoir, and in a subsequent paper on the pressures of 

 ether and other vapours (' Memoires de 1'Institut,' vol. 26), he adduces reasons in 

 * 'Annales de Chimie,' vol. 15, 1845, p. 129, and vol. 37, 1853, p. 257. 



