8 PRELIMINARY CONSIDERATIONS 



remain gaseous under the ordinary condition of the 

 atmosphere. 



The hygroscopic property of organic substances, 

 which is illustrated in the expansion or contraction of 

 the belts of sea-wrack which occasionally serve as 

 hygrometers, or in the shortening or lengthening of 

 a lady's ringlets, according to the moisture of the 

 atmosphere is another circumstance which may in 

 part account for the changes of volume in these tissues. 

 It is probable that the heat which determines the 

 evaporation of water from the earth operates, remotely, 

 in preparing these bodies to receive a portion of this 

 fluid within their interstices ; but whatever the expla- 

 nation may be, the phenomenon is of a physical 

 character, inasmuch as it is exhibited when the sub- 

 jects of it are removed altogether from the operations 

 of vitality. This hygroscopic property, therefore, is 

 another reason why the remarkable changes of volume 

 in the rudimentary tissues of the organism, under 

 slight changes of temperature, may be nothing more 

 than the natural and physical consequence of the 

 constitution of these tissues. 



Such would appear to be some general facts in con- 

 nexion with the operation of one form of external 

 agent, in some of the simpler phenomena of vital 

 movement. They show that the motions in the tissues 

 of the plant and animal, of which mention has been 

 made, may be referred in part to causes that are 



