Page:Popular Science Monthly Volume 17.djvu/828

808 of this strip I will warm in water, heated to 50° Cent., and the other I will stretch. 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 considerable rise of temperature. Here is a similar set of strips, which were stretched some hours ago, and which on trial by the air-thermometer we now find to have cooled down to the temperature of the surrounding objects. Note the effect of releasing the tension and allowing 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 elasticity, 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 exposure to cold for a moderate period. If, however, the action of heat has been pushed still further, say to 200 Cent., the caoutchouc becomes 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 hermetically 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 hydrocarbons, which present many points of interest, and you will find a