128 SOME APPLICATIONS OF 
“A body which when subjected to a given stress at a given tempera- 
ture experiences a strain of definite amount, which does not increase 
when the stress is prolonged, and which disappears completely when 
the stress is removed, is called a perfectly elastic body.” 
“Ifthe form of the body is found to be permanently altered when 
the stress exceeds a certain value, the body is said to be soft or plastic, 
and the state of the body when alteration is just going to take place 
is called the limit of perfect elasticity.” 
“Tf the stress, when it is maintained constant,causesastrain - -— - 
which increases continually with the time, the substance is said to be 
viscous.” 
A viscous material may be either solid or fluid. It is regarded by 
Maxwell as fluid when any stress, however small, produces a constantly 
increasing strain. Maxwell draws a distinction between elasticity of 
bulk and elasticity of shape—the latter being peculiar to solids—which 
is more fully treated of by Sir William Thomson. A body possesses 
perfect elasticity of bulk when on the removal of the stress it returns 
to its original volume, even though the form of its surface be perma- 
nently altered. Both writers regard it as certain that solid bodies will 
retain perfect elasticity of bulk under compressive stresses which far 
exceed the limit of elasticity of shape. The following statement em- 
bodies the views of Sir W. Thomson*: “If we reckon by the amount 
of pressure, there is probably no limit to the elasticity of bulk in the 
direction of the increase of pressure for any solid or fluid; but whether 
continued augmentation produces continued diminution of bulk toward 
zero without limit, or whether for any or every solid or fluid there is a 
limit toward which it may be reduced in bulk, but smaller than which 
no degree of pressure, however great, can condense it, is a question 
which can not be answered in the present state of science.” 
Maxwell, by denying the existence of a perfectly rigid body, main- 
tains that every solid can sustain stress or transmit force only by suf- 
fering strain. Thus, on depositing a feather on the most solid block of 
iron we produce in the iron a system of strains, infinitesimally small it 
is true, but whose existence can no more be questioned than the exist- 
ence across the surface separating the iron and the feather of forces 
balancing the portion of the feather’s weight left uncompensated by 
the air pressure. The hypothesis quoted above from Sir W. Thomson, 
that there may be a limit beyond which no body can be compressed, is 
not inconsistent with Maxwell’s statement. The hypothesis regards 
the ratio of the increment of strain to the increment of pressure as ulti- 
mately becoming infinitesimally small, but it in no way implies that 
this ratio ever becomes absolutely zero. 
In a solid bar, supposed perfectly elastic, exposed to longitudinal 
stress, the ratio of the stress to the strain is styled Young’s modulus. In 
many materials Young’s modulus varies in magnitude according to the 
s 
* Mathematical and Physical Papers, vol, 1, pp. 7-8, 
