546 



ORGANOLOGY. 



on Hygrometry (Philosoph. Transactions, vol. Ixxxi. Parts I. and II.), 

 distinguishes very minutely the double effect which humidity exercises on 

 hygroscopical substances, both of animal^ as of vegetable origin : viz., 

 Istly, the distension of the membrane or fibre itself, which invariably 

 takes place in both by the absorption of moisture ; and, 2dly, the con- 

 traction which takes place in both of entire portions (especially of spiral 

 ones) by water getting between the separate fibres (or between the cell- 

 walls), which are thereby bent, and thus far produce a contraction of the 

 individual part, notwithstanding the simultaneous distension of the mem- 

 brane. The phenomena of hygrometrical substances are connected with 

 both causes, and the sum total of the result must be exhibited according 

 to the predominance of one or the other of these causes, either as a 

 distension or a contraction. How the relations vary in this respect will 

 be shown by the following table from DeLuc, which proves, at the same 

 time, that all vegetable, as well as animal substances, can be distended by 

 moisture. The second effect, however, begins to manifest itself at 100, 

 and a gradual contraction then takes place also in animal substances. 



Table of the relative Rates of Humidity in different Fibres of Vegetable and Animal 

 Substances, taken longitudinally. 



Next to the boxwood (cut longitudinally) a twisted hemp rope ought 

 to be enumerated, in which, in consequence of the close junction of the 

 fibres, the second effect takes place still earlier. This is the result of 

 scientific research upon this subject ; and Link's assertions to the con- 

 trary are the result of sheer ignorance. 



It is a very common thing to hear general phrases made use of re- 

 specting hygroscopicity as the result of desiccation, &c., without any 



