156 DR. FRANCIS DARWIN ON THE MECHANISM BY WHICH 
tremely sensitive *; but as we shall see that it is also very sensitive to changes of tempe- 
rature, a purely hygroscopic result is not so strikingly demonstrable as one due to a change 
in temperature and in moisture combined. Thus, breathing on the Stipa-hygroscope, 
held even at arm's length, or bringing the warm and moist hand near the awn, causes 
distinct movements of the index. 
De Lue showed t that water has no special virtue in causing the expansion of hygro- 
scopie bodies, and that other fluids can perform the office of filling out the molecular 
interstices. If the Stipa-hygroscope is allowed to dry in the air of an ordinary room, 
and is then plunged into absolute alcohol, the index shows, by moving in the “ dry” 
direction, that water is being removed from the awn; after a time the alcohol itself is 
absorbed, and the movement of the index is reversed. On removing the instrument, the 
index moves in the “wet” direction, in consequence, І suppose, of the absorption of 
water by the alcohol; ultimately, however, the alcohol evaporates, and the movement of 
the index is reversed. 
The effects of temperature on the awn are extremely curious, and agree with those 
observed by De Luc 1: he found that a rise in temperature affects hygroscopic bodies in 
the same way as increased moisture, whereas a fall in temperature acts like dryness. 
With a Stipa-hygroscope the experiment is a pretty one; it is allowed to remain in warm 
water until the index comes to rest; if it is then removed, and plunged into cold water, 
the index gives a quick start through 90° or more in the direction of drying (2.6. against 
the hands of a watch) On replacing it in the warm water a rapid movement in the 
opposite, or “ wet," direction takes place. Тһе experiment may be performed in another 
way. Ifthe hygroscope is placed in the current of hot air from a lamp, the rise in tem- 
perature tends to make the index move in the wet direction; but it also dries it, and 
therefore tends to make it rotate in the opposite direction: the struggle is made obvious 
by the fluttering motion of the index. Тһе ultimate result is the victory of the * drying" 
rotation. The coincidence in the effects of heat and moisture accounts for the extreme 
sensitiveness of the S¢ipa-hygroscope to being breathed on. 
The first theory which suggests itself to account for the effects of temperature is, that 
as an increase in the quantity of water in the molecular interstices makes the awn 
untwist, an increase in bulk of the water already permeating the tissues acts in the same 
manner. In support of this view my brother Horace showed that it cannot be due to 
air contained in the tissues; for the index does not move when the receiver of an air- 
pump under which it is placed is exhausted, or when the air is readmitted. In per 
forming this experiment we immersed the hygroscope in oil or in mercury, to obviate 
the effects of the change of temperature resulting from the change of air pressure; 
before this precaution was taken the index moved distinetly as the air was removed or 
let in, although the thermometer only registered a change of 2° F.$ 
* It has been employed as a hygrometer (Watts, Dict. of Chemistry, iii. p. 233). 
+ Phil. Trans. 1791, p. 11. t Loc. cit. p. 16. 
$ The sensitiveness to changes in temperature was well exhibited by my brother, who observed that on lifting the 
Stipa-hygroscope out of water it always moved very slightly in the cold direction, but that this movement disappeared 
at the temperature of the dew-point, showing that it was due to evaporation. 
