INTO THE STRENGTH OF WROUGHT-IRON PLATES. 
687 
Many of the fibres are stretched, but only to the extent of bringing- the others to bear 
upon the load, which done, their united force constitutes the maximum of resistance 
to a tensile strain. 
Other bodies of less ductility and more of a crystalline structure, such as cast iron, 
stone, glass, &c., seem to be subject to the same law. In these cases it seldom happens 
that the whole of the particles are brought into action at once, as much depends upon 
the conditions of the body, the unequal state of tension of its parts, and the strain 
which some of the particles must sustain before the others receive their due portion 
of the load. Should the non-resisting particles be within the limits of elongation of 
the other particles, the body will then have attained its maximum power of resistance ; 
but in the event of rupture to any of tbe resisting particles, the cohesive force of the 
body is thereby reduced, and that to the extent of the injury sustained by the frac- 
tured parts. 
“There are however,” as Dr. Robinson truly observes, “immense varieties in the 
structure and composition of bodies which lead to important facts, and prove that 
the absolute cohesion of all bodies, whatever be their texture, is proportional to the 
areas of their sections.” Undoubtedly this is the case in bodies having an uniform 
texture with straight fibres, and hence it follows that the absolute strength of a body, 
resisting a tensile strain, will be as the area of its section. 
The peculiar nature of the material combining a crystalline as well as a fibrous 
structure has led to these observations. In some instances the specimens experi- 
mented upon exhibited an almost distinct fibrous texture, and in others a clearly de- 
veloped crystalline structure*. At other times some of the specimens were of a mixed 
kind, v/ith the crystalline and fibrous forms united; the fracture having a laminated 
appearance, with the crystalline parts closely bound on each side by layers of the 
fibrous structure. These varieties are probably produced in the manufacture, and 
may be easily effected either by the mode of “piling” the layers of bars which form 
the plate, or from the unequal temperature of the parts as they pass through the rolls. 
But whichever way they are produced, it is evident, from the experiments, that the 
fractures gave, in most cases, indications of an unequal and varied texture. 
In the foregoing experiments, and also in those which follow, great attention was 
paid to the appearance of the fracture, in order to ascertain the structure of the 
plate, and to determine how far it could be depended upon in its application to the 
varied purposes for which it was intended. 
These appearances are all shown in the drawings appended to the experiments, and 
to which I beg to refer. 
* See the fractured parts of the different specimens, Plate LV. 
