Al4 On the Resistance of Solids. 
When the particles of a body can be moved only in a very small 
degree, without destroying their cohesion, the body is called a sold. 
2. The absolute cohesion of solids is measured by the force 
necessary to pull them asunder. Thus, if a rod of iron be sus- 
pended in a vertical direction, and weights be attached to its 
lower extremity till the red breaks, the whole weight attached 
to the rod, at the time of fracture, would be the measure of its 
cohesive force, or absolute cohesion. 
3. The particles of solid bodies, in their natural state, are - 
arranged in such a manner that they are in equilibrio in re- 
spect to the forces which operate on them, Therefore, when 
any new force is applied, it is evident that the equilibrium will 
be destroyed, and that the particles will move among themselves 
till it be restored. 
When the new force is applied to pull the body asunder, the 
body becomes longer in the direction of the force ; whieh is 
called the extension ; and its area at right angles to the direc- 
tion of the force contracts. 
When the force is applied to compress the bedy, it becomes 
shorter in the direction of the force, which is called the com- 
pression; and the area of its section, at right — to the force, 
expands*. 
In either case, a part of the heat, or any other fluid, that oc- 
cupied the pores or interstices of the body, before the new force 
was made to act upon it, will be expelled }. 
4, The depth of a beam, or bar, is the dimension in the di- 
rection of the pressure. 
Phenomena. 
5. All bodies, as far as experience reaches, are extended or 
compressed by an adequate force. 
The extensibility of the most brittle bodies may be rendered 
sensible by forming them into thin plates. Plates of glass bend 
considerably when they are only supported at the ends; and this 
bending could not take place unless the body were both com- 
pressible and extensible. Marriotte succeeded in extending some 
* The resistance to compression does not arise from any repulsive power 
in the particles of bodies ; indeed, we have not any facts to prove that the 
particles of any class of bodies actually repel each other ;—as, whenever a 
body is forced into a less space than it occupies in its natural state, one of 
its constituents, at least, is always expelled; and is restored by a species of 
capillary attraction, as soon as the external force i is removed. 
+ The disengagement of heat in experiments on the direct cohesion of 
bodies, appears to haye been first noticed by Perronet in his experiments 
on the strength of iron. Mr. Barlow has lately observed the same phzeno- 
mena in a greater degree, owing to the large size of the bar that was ex- 
perimented upon—(Ann. of Phil. x.311)—as it is obvions that the quantity 
of heat disengaged must*be proportional to the magnitude of the body. 
Callies 
