8o8 THE POPULAR SCIENCE MONTHLY. 



of this strip I will warm in water, heated to 50 Cent.,' and the other 

 I will sti-etch. In each case you see that the caoutchouc is restored 

 to its original condition. In the case of the stretching it is very- 

 likely that the effect is due to the heat evolved during that operation. 

 It is easy to illustrate the fact that heat is produced when India-rubber 

 is subjected to tension. Here are some strips of India-rubber, arranged 

 side by side on a board. I bring them in contact with the bulb of an 

 air-thermometer, and you see that there is no indication of either heat 

 or cold. The strips of India-rubber being now stretched to four or 

 five times their previous length, the air-thermometer indicates a con- 

 siderable rise of temperature. Here is a similar set of strips, which 

 were stretched some hours ago, and which on trial by the air-ther- 

 mometer we now find to have cooled down to the temperature of the 

 surrounding objects. Note the effect of releasing the tension and al- 

 lowing the rubber strips to contract. You see that they have become 

 so cold as to influence the air-thermometer to a very considerable 

 extent. 



The effects of heat on India-rubber present many points of interest, 

 and, in the first place, I wish to illustrate to you the effect of moderate 

 heat on a stretched band of caoutchouc. Here is such a band, one end 

 being attached to an index, pointing, at the present time, to the zero 

 of this paper scale. Notice the consequence of applying a gentle heat 

 to the caoutchouc band it contracts as regards its length, but expands 

 in a transverse direction, causing the index to move rapidly through a 

 space of several degrees. This property, which stretched caoutchouc 

 possesses, of contracting by heat, may be described by saying that, 

 within certain limits, the tensile elasticity of caoutchouc is increased 

 by an elevation of temperature. Caoutchouc, however, if heated to 

 100 Cent., softens considerably, and almost entirely loses its elas- 

 ticity, as you will perceive by examining this sample, which has been 

 heated for some hours ; while a heat of 120 Cent, produces a most 

 decided softening effect on caoutchouc of the best quality, but after 

 exposure to this temperature, it recovers its pristine state by expos- 

 ure to cold for a moderate period. If, however, the action of heat 

 has been pushed still further, say to 200 Cent., the caoutchouc be- 

 comes converted into a permanently viscous body, which has little or 

 no tendency to harden again. This viscous substance possesses the 

 same composition as unaltered caoutchouc, and is of value as a medium 

 for making air-tight joints, which can be easily undone. This glass 

 jar has its top edge ground level, and, after applying a little of the 

 heated caoutchouc to the ground edge, the jar may, as you see, be her- 

 metically closed by a disk of plate-glass. A joint of this kind may be 

 broken and remade with the utmost facility and rapidity. 



When caoutchouc is subjected to a temperature somewhat above' 

 200 Cent., it becomes converted into a variety of volatile hydrocar- 

 bons, which present many points of interest, and you will find a toler- 



