760 
The nearly-vertical lines on each curve, 
at intervals, from the origin, are the 
‘elasticity lines,’ as they have been called, 
showing the action of the material when 
the load is gradually relieved. These 
substances do not restore themselves to the 
original dimensions, but remain perma- 
nently distorted, taking more or less set and, 
usually at least, maintaining a constant 
value of the modulus of elascity. The 
slight droop in each line, near its outer ex- 
tremity, shows the behavior of the sub- 
stance when the load is left unchanged and 
elongation is maintained constant. This 
“exaltation of the elastic limit’ was discov- 
ered by me many years ago, and announced 
to the American Society of Civil Engineers 
in 1873.* India rubber exhibits none of 
the phenomena giving the characteristic 
form of the diagrams of the irons and steels. 
Even when stretched to the point of rup- 
ture it restores itself very nearly to its 
original dimensions and gradually recovers 
a part of the loss of form at that instant 
observable. Its almost complete stability 
of form when relieved from load, and es- 
pecially when in the shape of springs such 
as are used on railway trucks, constitutes 
one of its most valuable properties. Like 
cork, when confined laterally, it is practic- 
ally indestructible and incapable of distor- 
tion when used as a spring. The singular 
stress-strain relations of the substance may 
probably be found to give it peculiar value 
for many other purposes. The sample il- 
lustrated by our diagram was of the kind 
employed for springs and rubber bands and, 
as usual, slightly vulcanized. 
The equation of the curve of this charac- 
ter described by the materials of construc- 
tion generally may be taken, without im- 
portant error, usually, as of the form 
E=a/T : 
and with elongation, H, and T, tenacity, re- 
* Trans. Am. Soc. C. E., November, 1873. 
SCIENCE. 
[N.S. Von. VI. No. 151. 
spectively, in per cent. of total length of 
specimen and in pounds per square-inch, the 
constant, a, is usually not far from 0.1 for 
good irons, 0.05 for tool-steels, and for good 
soft copper itisabout1.2. For this specimen 
of india rubber the equation is obviously 
so entirely different in class and form, and 
indicates so evidently an entirely different 
method of molecular action in resistance of 
strain, that it must constitute a problem by 
itself. The peculiarities of this form may, 
however, prove a key to the corresponding 
singularities of its internal structure and 
forces. So faras known, no other substance 
gives such unique relations of forces holding 
the substance in stable form to the varia- 
tions of form to which the external applica- 
tion of force givesrise. The volume of the 
mass remains, so far as can be seen, con- 
stant, or nearly so, and the expenditure of 
work upon the substance results simply in 
changing the intermolecular distances of 
adjacent particles. This fact will probably 
be found to simplify the process of experi- 
mental interpretation of this curious case. 
The cause of the singular behavior of 
this thermodynamically unique material re- 
mains to be discovered. Itis probably safe 
to assume that its mechanical and physical 
properties will have some close relationship, 
and that the investigation, to be complete, 
must comprehend both lines of research. 
The gum must evidently possess either some 
very strange molecular force-relations or it 
must be the fact that, in the case of the or- 
dinary materials of engineering and con- 
struction, these apparently peculiar char- 
acteristics are, in them, as yet unobserved 
because of their minute effects. Thestudy 
of the subject cannot fail to lead to new and 
interesting facts relative to the molecular 
constitution of this and probably of other 
substances. 
R. H. THurston. 
CORNELL UNIVERSITY. 
