the Laws of Evaporation and Absorption. 285 



into tenths of an inch, so that with a vernier the contraction or 

 elongation, as the case might be, could be read off to hundredths 

 of an inch. After the strips had become dry, they nearly re- 

 turned to their original lengths. 



A curious experiment. — Attach an oblong plate of metal to 

 one extremity of a cotton thread, and suspend it from a loop 

 formed in the other extremity : immerse the thread in a deep 

 glass jar filled with water; then the plate will revolve rapidly 

 in a direction contrary to the direction of the twist of the thread : 

 the rotation will go on for some time after the thread has been 

 taken out of the water. When the rotation has ceased, let the 

 plate be restrained from any further rotation; then after the 

 thread has become dry, remove the obstruction placed against 

 the plate, and it will again rotate in the same direction, and not, 

 as might have been expected, in a direction contrary to that in 

 which it had at first revolved. The cause of this rotation I appre- 

 hend to be as follows : — The moisture, by causing the thread to 

 contract, thereby tends to tighten the twist of the thread, and, 

 as a necessary consequence, the thread tends to revolve in a 

 direction contrary to that of the twist. 



Spontaneous absorption of moisture from the atmosphere by dif- 

 ferent absorbents. 



If an absorbent be thoroughly dried and then exposed to a 

 humid atmosphere at or near to the dew-point, it will gradually 

 absorb moisture from the air. The moisture thus absorbed by 

 some substances is something considerable. Woollen cloth 

 absorbs one-seventh of its own weight of moisture from air 

 whose temperature is about one degree above that of the dew- 

 point; bran about one- eighth of its weight; calico about one- 

 tenth; and so on. The amount of absorption increases as the 

 temperature of the air approaches that of the dew-point : the 

 maximum quantity of moisture absorbed takes place in an atmo- 

 sphere saturated with the vapour of water. 



1. The rates of absorption by different absorbents presenting 

 equal surfaces are proportional to their respective maximum 

 quantities of absorption. Moreover, the moisture absorbed by 

 two equal surfaces of the same material, but of different thick- 

 nesses, is proportional to their respective weights. 



Thus two equal surfaces of black cloth and flannel, weighing 

 respectively 400 grains and 300 grains, absorbed during the 

 same time of exposure 41 and 32 grains respectively, and their 

 maximum quantities of absorption were found to be 60 and 47 

 grains of moisture respectively. Here we have — ratio of 



300 32 



weights = jp^ ='75; ratio of rates of absorption __ j^ ='78; 



