SECT, xiv.] CONSTITUTION OF BODIES. 119 



In elastic substances, on the contrary, its action is weak, 

 but more extensive. Since all bodies expand by heat, the 

 cohesive force is weakened by an increase of temperature. 



Every particle of matter, whether it forms a constituent 

 part of a solid, liquid, or aeriform fluid, is subject to the law 

 of gravitation. The weight of the atmosphere, of gases and 

 vapour, shows that they consist of gravitating particles. 

 In liquids the cohesive force is not sufficiently powerful to 

 resist the action of gravitation. Therefore although their 

 component particles still maintain their connexion, the 

 liquid is scattered by their weight, unless when it is confined 

 in a vessel or has already descended to the lowest point 

 possible, and assumed a level surface from the mobility of 

 its particles and the influence of the gravitating forces as in 

 the ocean, or a lake. Solids would also fall to pieces by the 

 weight of their particles, if the force of cohesion were not 

 powerful enough to resist the efforts of gravitation. 



The phenomena arising from the force of cohesion are in- 

 numerable. The spherical form of rain drops ; the difficulty 

 of detaching a plate of glass from the surface of water; the 

 force with which two plane surfaces adhere when pressed 

 together; the drops that cling to the window-glass in a 

 shower of rain are all effects of cohesion entirely indepen- 

 dent of atmospheric pressure, and are included in the same 

 analytical formula (N. 158) which expresses all the circum- 

 stances accurately, although the laws according to which 

 the forces of cohesion and repulsion vary are unknown. It is 

 more than probable that the spherical form of the sun and 

 planets is due to the force of cohesion, as they have every 

 appearance of having been at one period in a state of fusion. 



A very remarkable instance of cohesion has occasionally 

 been observed in plate-glass manufactories. After the large 

 plates of glass of which the mirrors are to be made have 

 received their last polish, they are carefully wiped and laid 

 on their edges with their surfaces resting on one another. 

 In the course of time the cohesion has sometimes been so 



