J (So MOLECULAR FORCES IN THE PLANT. 



These facts, in connection witli a number of other phenomena, first led Niigeli 

 to the hypothesis that organised bodies consist of isolated particles or Molecules 

 between which the water penetrates, and which are solid and relatively unchange- 

 able, and invisible even with the most powerful microscopes. Every molecule of a 

 saturated organised body is, on this hypothesis, surrounded by layers of water by 

 which the adjacent molecules are completely separated from one another. These 

 molecules may be supposed to be of various sizes, and it is evident a priori that, if 

 the thickness of the aqueous envelope is the same, larger molecules will form a 

 denser, smaller molecules a less dense substance ; and it may therefore be concluded 

 conversely that the layers and lamellae of organised bodies of different thickness, 

 especially those of the cell- wall and of starch-grains, are composed of molecules of 

 different sizes ; and the difference in the proportion of water in such cases leads to 

 the hypothesis that the densest substance consists of molecules which are several 

 thousand times larger than those of the more watery substance. As the molecules 

 increase in size, the density of the whole substance is moreover increased by the 

 smaller distance that intervenes between them, so that larger molecules are separated 

 from one another by thinner layers of water. The changes in volume of organised 

 bodies due to the removal of water or its absorption, depend, according to this view, 

 on the fact that when swelling takes place the molecules are forced further apart by 

 the water which penetrates between them ; while, on the other hand, when water is 

 removed they approach one another in proportion as the water is withdrawn from 

 their interstices. 



The forces which are concerned in these processes in the interior of an organ- 

 ised body may be divided into three kinds :— (i) the Cohesion within each separate 

 molecule impermeable to water, which is itself an aggregate of smaller molecules 

 and atoms; (2) the Attraction of the adjoining molecules for one another, in 

 consequence of which they tend mutually to approach; and (3) the Attraction of 

 the surfaces of the molecule for the absorbed water, which counteracts the mutual 

 attraction of the neighbouring particles. 



In starch-grains, cell-walls, and to a certain extent in crystalloids \ the absorbed 

 water is not deposited uniformly in all directions ; the molecules are, on the con- 

 trary, forced further from one another in certain directions, as is clearly seen from 

 the change of form of the whole, from the formation of fissures, &c. One of the most 

 remarkable effects of the tensions thus caused in the interior of the body is the fact 

 that when swelling takes place particular dimensions may even decrease ; thus, for 

 example, the layers of bast-fibres become very considerably shorter when they sv,^ell 

 up under the influence of dilute sulphuric acid, the coils of the spiral striation be- 

 coming closer and larger in circumference. Crystalloids change their angles several 

 degrees when they swell. These phenomena are explicable only on the supposidon 

 that the molecular forces in the interior of organised substances vary in intensity in 

 different directions ; and this again is conceivable only on the hypothesis that the 

 form of the molecules is not spherical. Nageli and Schwendener obtained a deeper 

 insight into these laws by a very careful observation of the phenomena produced by 



^ [See Ijook I, pp. 40-57, on Crystalloids.] 



