CH. IV] STIPA HYGROMETEK. 103 



the Phil. Trans, for 1898 \ The horn-hygroscope is shown 



in fig. 20, where c is a block of cork on the lower surface 



of which is cemented a strip cut from a 



shaving of pressed horn, T, which carries 



at its free end a bristle, 6, to serve as 



an index : a cardboard quadrant G is 



cemented to the vertical face of the 



FIG. 20. Exp. 118. 

 cork block and is graduated as shown. 



If the hygroscope is placed on a dry surface the index 

 remains at zero, but if on a surface from which water 

 vapour is coming off, e.g. the stomatal side of a leaf, the 

 tongue of horn at once curves away from the source of 

 moisture so that the index travels over the graduations. 



The only difficulty connected with the hygroscope is 

 the preparation of the horn. The thinnest possible 

 shavings, cut across the grain by means of a lathe, from 

 pressed and heated horn, are needed. From these we 

 select the best pieces which are wetted with distilled 

 water and spread out flat between two glass plates 

 clamped together. The horn is thus kept flat while it 

 is cautiously heated over a gas flame 2 . When horn 

 cannot be obtained, the material (gelatine ?) from which 

 hygroscopic toys, in the shape of fishes, etc., are made, 

 may be used : but it is not nearly so durable as 

 horn. 



In using the instrument it is advisable only to leave 

 it on the leaf for a few seconds at a time, otherwise the 



1 Francis Darwin, 'Observations on Stomata,' Phil. Trans. B 165, 

 1898. 



2 The Scientific Instrument Company, Cambridge, usually lias a 

 stock of suitable horn. 



