110 
DR. JOULE ON SOME THERMO-DYNAMIC PROPERTIES OF SOLIDS. 
Interval of time between the 
successive determinations. 
Tension on the rod, 
in pounds. 
E.xpansion 
per degree Centigrade. 
A few hours 
35 
•000004612 
Two or three days 
435 
•000005665 
A few liours 
35 
•000004372 
Two or three days 
435 
•000005685 
A few hours 
35 
•000003952 
•000004699 
Two or tiiree days 
435 
A few liours 
35 
•000004003 
435 
•000004850 
Mean for 
35 
•000004235 
Mean for 
435 
•000005225 
63. The above experiments manifest most strikingly the effect of tension in increasing 
the expansibility of wood, that with 435 lbs. being nearly one quarter greater than that 
with only 36 lbs. The gradual diminution of the coefficient perplexed me a good deal 
at first, particularly as the mercury in contact with the wood prevented the access of air 
to its sides, but I was ultimately able to refer it to the gradual absorption of moisture 
through the pores in the direction of the length of the rod. 
64. The rod was now exposed to a tension of 435 lbs. for twenty-two days, at the end 
of which time its expansion by heat was found to be ‘000004784. The weights were 
then removed so as to reduce the tension to 35 lbs. After remaining two days in this 
state, the first effect was found not to be expansion, but, on the contrary, a contraction 
amounting to ‘000002408 per degree. Then the subsequent application of cold pro- 
duced a contraction of ‘000005705. On raising the temperature again after an interval 
of five minutes I found an expansion of ‘000002170, and then on cooling, a contraction 
of ‘000005017. After twenty minutes I found that the expansibility by heat, indi- 
cated by the mean effect of raising and depressing the temperature alternately, was 
•000003*575. 
65. From the above it appears that wood has to a certain extent the property pos- 
sessed by india-rubber, of returning from a state of strain on the application of heat. 
Mr. Hodgkinson has obsei’ved this property in iron ; I have noticed it in whalebone, 
hail', leather, and to a slight extent in copper. 
66. The increase of expansibility with tension suggested that the force of elasticity of 
wood decreases with a rise of temperature. To try this I affixed a graduated glass plate 
to the top of the glass tube (see fig. 7), while parallel and close to it another divided 
glass plate was affixed to the wooden rod. The extension of the rod under various ten- 
sile forces was thus read off by means of a microscope in -^oths of an inch and fractions 
of the same appreciable to -^th of the actual divisions. The mean of several series of 
trials gave me, at a low and high temperature, the following indications of the micro- 
meter, with stretching weights increased by 100 lbs. at a time until 435 lbs., and then 
decreased by the successive removal of the weights again. 
