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under the microscope, of a combination of solid material with 

 water ; but when the cell-wall is very thin, no microscopic 

 observations can show that the cell-wall has any particular 

 structure, and it appears to be a perfectly homogeneous mem- 

 brane. If these organized structures are placed in, or treated 

 with, a reagent such as alcohol, which is capable of removing 

 water, a part of their aqueous contents is withdrawn, and con- 

 sequently they diminish in size ; while on the other hand, 

 when treated with certain reagents, such as dilute acids and 

 alkalies, they have the peculiar property of swelling up — that 

 is, of absorbing a certain amount of fluid, with a consequent 

 increase of bulk. From these phenomena Nageli inferred that 

 the starch grain, for example, consists of isolated solid particles, 

 which are themselves impenetrable by water, but which are 

 capable of taking up a certain amount of water between them, 

 and that the amount of this water may vary according to cir- 

 cumstances. The swelling up, then, depends on the taking up 

 of a certain quantity of water, and the temporary retention of 

 the same between the particles of the cell-wall ; and upon this 

 property of swelling up the growth in size, and the peculiar shape 

 of the cell depend. Within certain limits, these variations in 

 the proportion of water to solid matter may occur without 

 occasioning any permanent change in the molecular structure ; 

 but if, with a higher temperature, and in presence of chemical 

 reagents, the proportion falls below a certain minimum or 

 exceeds a certain maximum, permanent changes in the internal 

 structure take place, which can no longer be reversed, and the 

 internal organization of the body becomes partially or entirely 

 destroyed. 



Every molecule of a saturated organized body is, on this 

 theory, surrounded by layers of water, by which the adjacent 

 molecules are completely separated from one another. These 

 molecules are solid and relatively unchangeable, and invisible 

 even with the highest attainable microscopic powers. We may 

 compare the whole cell- wall to a brick wall or to a regiment of 

 soldiers seen at a distance, which, on closer inspection, we find 

 to be composed of smaller particles, viz., bricks or men. When 



