THE 



PHILOSOPHICAL TRANSACTIONS 



OF THE 



ROYAL SOCIETY OF LONDON; 



ABRIDGED. 



/. A Second Paper on Hygrometry. By J. A. De Luc, Esq., F. R. S. Anno 



1791. Fol. LXXXL p. 1. 



In a paper which Mr, D. presented to the r. s. in the year 1773, he sketched 

 the following propositions, as fundamental for the construction of an hygrometer. 

 1st. That fire, considered as the cause of heat, was the only agent by which ab- 

 solute dryness could be immediately produced. 2d. That water, in its liquid 

 state, was the only sure immediate means of producing extreme moisture in 

 hygroscopic bodies. 3d. That there was no reason, a priori, to expect from any 

 hygroscopic substance, that the measurable effects produced in it by moisture 

 were proportional to the intensities of that cause ; and, consequently, that a 

 true hygrometrical scale was to be a particular object of inquiry. 4th, lastly, 

 That perhaps the comparative changes, of the dimensions of a substance, and 

 of the weight of the same or other substance, by the same variations of mois- 

 ture, might lead to some discovery in that respect. The same propositions are 

 the subject of this paper. Accordingly, it first treats of absolute dryness ; 

 stating that an hygroscopic body, which is not brought into contact with any 

 other body drier than itself, cannot lose any part of its moisture but by evapora- 

 tion ; and if this is entirely produced by fire, there may be such a degree of 

 heat as will cause the total evaporation of that moisture. It next treats of ex- 

 treme moisture, being the 2d proposition sketched in the first paper, viz. that 

 water, in its liquid state, is the only sure immediate means of producing extreme 

 moisture in hygroscopic bodies. On this head Mr. D. relates several experi- 

 ments, from which he infers as follows ; from the whole of the foregoing experi- 

 ments there cannot remain any doubt, that water, in its liquid state, is a sure 

 means of fixing the point of extreme moisture on hygrometers. Particularly, in 

 respect of elastic substances, as ivory, quill, whalebone, all sorts of wood, and 

 a number of others which have been tried ; the last experiments in water of dif- 

 ferent temperatures, afford an immediate proof, that their faculty of sucking 



vol.. XVII. B 



