Day — Residiial Viscosity on Thermal Exjpansion. 345 



brackets may be clamped in any position along tlie bar, aa, by 

 means of thumb-screws and projecting jaws, and their form is 

 such that they successfully resist the slight force tending to 

 separate their lower extremities. 



Since the portion of the bar aa^ between the brackets is con- 

 stant in length, owing to the stream of water passing through, 

 it is readily seen that a base of constant length is provided, and 

 that, too, in a simple and effective manner. 



Moreover, by means of the movable brackets, the apparatus 

 may be accommodated to solids varying widely in their coeffi- 

 cients of expansion. Thus, the apparatus permitted the testing of 

 a standard meter rod for temperature corrections, while, in the 

 experiments with hard rubber, which are referred to above, a 

 piece of this substance but IS'^"' in length, which was sufficient 

 to give conveniently large scale-deflections, was used, and the 

 apparatus was readily adjusted to this length of rod. 



The data obtained with metal rods are of no special interest, 

 except as they point out the degree of accuracy of the results 

 obtained with the apparatus ; at the same time the action of 

 the apparatus is well enough shown in the results obtained with 

 the hard rubber rod, which are exhibited as expansion curves 

 in the accompanying diagram, fig. 3. 



A piece of hard rubber tube was examined by means of this 

 apparatus, to determine the range of temperature through 

 which it would maintain a uniform rate of expansion, with 

 results as shown by the curve, suv-wz. The rubber maintained 

 a nearly uniform rate up to 50", when an apparent rapid con- 

 traction took place until 61° was reached. At this point the 

 bath was allowed to cool off several degrees and then was raised 

 to 64°, giving the short, detached portion of the curve, wz. 

 Evidently, the apparent contraction went on during the cooling 

 and reheating of the bath below 61°. This fact gives a clue to 

 the real cause of the apparent contraction ; that it was no real 

 contraction due to a rise in temperature, such as is shown by 

 rubber under certain well-known conditions, but a viscous 

 readjustment of the molecules, after being released by the heat 

 of the bath from a condition of stress due to the previous treat- 

 ment of the rubber, — probably, in its manufacture. 



A subsequent test of the same piece of rubber gave further 

 evidence of this condition of residual viscosity, as is shown by 

 the curve, ahce. The rubber, as before, maintained a uniform 

 rate up to 50°. From this point up to 64°, a gradual dimi- 

 nution of the rate is observed. At 64°, the highest tempera- 

 ture to which the rubber was previously subjected, a rapid 

 decline is noted. 



These results clearly show that the rubber is molecularly 

 stable at a given temperature, if it has long been exposed to 



