VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 26j 



tain quantity of water, are thereby expanded ; and these are most of the solids 

 belonging to the vegetable and animal kingdoms. Various hygroscopic pheno- 

 mena, which only depend on the different properties of the substances themselves, 

 being thus foreign to the fundamental laws of hygrometry, Mr. D. here confines 

 himself to the last class, which appears the only proper one for that general pur- 

 pose ; and, among the hygroscopes of that class, he only considers those which 

 cease to lengthen, only when they cannot be penetrated with more water. 



Moisture, taken in a general sense, may be considered simply as invisible 

 water, producing observable phenomena. Thus, in hygroscopic bodies, the 

 quantity of water which expands them, and increases their weight, is concealed 

 within their pores ; and in the ambient medium, that water which affects hygro- 

 scopic bodies, being there under the form of steam, is as invisible as air itself. — 

 But in respect of hygrometry, where moisture is considered as having correspond- 

 ent degrees in the medium, and in hygroscopic substances, that word requires a 

 more particular determination, on account of those two different situations of in- 

 visible water. Moisture may be either totally absent, or absolutely extreme, both 

 in hygroscopic bodies, and in the ambient medium ; which circumstance, on both 

 sides, affords a fixed module for determining correspondent degrees ; but these 

 modules are not of the same nature ; and thence, in their relation to each other, 

 both in the whole and in correspondent parts, moisture assumes in the mediqm, 

 the character of a cause, and in hygroscopic bodies, that of an effect. 



But are we permitted to con^sider the variations of the hygroscope as propor- 

 tional to those of moisture in the medium ? This, according to the above deter- 

 minations, would be the case, if the hygroscopic substance of the instrument 

 lengthened in proportion to the quantity of water that it may retain in the me- 

 dium. But the cause of the expansion of those substances by water, and the 

 capacity of their pores at different periods of moisture, are too complicated for 

 answering that question a priori ; and by experience, the great differences ob- 

 served in the marches of many of those instruments made of different substances 

 prevents us from assigning that property to any of them, till some particular ex- 

 periment comes to help us in that respect. However, that circumstance affects 

 only the practical part of hygrometry, and is foreign to the fundamental princi- 

 ples of that science. Mr. D. indicated, in his last paper, 2 means which he had 

 formerly imagined, for obtaining that desirable and still wanting correspondence 

 between the march of a determined hygroscope, and that of moisture in the me- 

 dium. One of those means was, to observe, at the same time, the variations in 

 weight and length of the same substance, in order to compare the quantities of 

 water which it retains, with their effects on its length. He has executed that 

 experiment ; but its results, given in his last paper, have confirmed his doubts, 



