between the lips and moving the under-lip to one side. There are apparently two pencils.

The distance between two points on the skin seems greater when the skin between these points is also touched. If four pins are pounded in a straight line into a stick at one-fourth of an inch apart, the distance between the end pins will appear greater than that between two separate pins three-fourths of an inch apart.

The distances apart of the various points that we feel are what we know under the names of smoothness and roughness. A billiard-ball is "smooth," that is, our sensations of touch are evenly distributed. Carpet is "rough," that is, it produces uneven sensations. Sandpaper is peculiarly "rough," because very intense and limited sensations from the sharp sand are mingled with smoother ones and gaps. Velvet, when felt backward, has a peculiar rough smoothness, because the separate points of the individual hairs produce separate sensations, yet are so near together as to resemble smoothness. Shortnap plush has a similar feeling.

IN

CHAPTER IX

HOT AND COLD

N the old days it was supposed that heat and cold were two different things; even to-day the everyday person cannot grasp the idea that coldness is simply the absence of heat, that a piece of ice is cold simply because it is not hot. But the modern development of physics has shown that heat consists of motion among the little molecules of which all bodies are supposed to be composed, and that as this motion becomes less the bodies are said to be cold. Thus a glass of warm water differs from a glass of cold water simply in the fact that the molecules of the water in the former are moving rapidly, while in the latter they are comparatively quiet.

Strange as it may seem, it was discovered a few years ago that the ordinary common sense of everyday people was right. Not that the science of physics was wrong, but that the conclusion drawn was incorrect. Hotness and coldness are two entirely different things from our point of view. A glass of water is warm because it gives us a feeling, or sensation, of warmth; another glass is cool because it gives us an entirely different sensation of coldness. The complete distinction of our feelings of hotness and coldness from the physical condition of the molecules

of the object touched is emphasised by an experiment in which the same object feels both hot and cold at the same time (see below).

Our sensations of hot and cold come from little spots called hot spots and cold spots. To find the cold spots a pointed rod, eg., a lathe centre, a pointed nail, or even a lead pencil, is cooled and then moved slowly

spot Map.

and lightly over the skin. At certain points distinct sensations of cold will flash out while elsewhere nothing but contact or vague coldness is felt. These points are the cold spots; a specimen arrangement of them is shown in Fig. 69.

To find the hot spots the metal point is FIG. 69-A Cold- warmed and applied in a similar manner. The hot spots are everywhere different from the cold spots. A specimen case is shown in Fig.70. At the art store get a few pounds of plaster for casts (the finely ground plaster, not the ordinary plaster of Paris). Mix it with water in a bowl. Pour out a portion into a tin pie-plate. Now press the hand (palm or back) down upon it, being careful to touch the plaster at every point. When the plaster has hardened sufficiently to permit the removal of the hand without sticking, carefully raise it. A perfect cast of the hand is obtained with every line expressed.

Now prepare yourself with a glass of ice water, a glass of hot water, some red

H

and some blue ink, a pointed metal FIG. 70-A Hot

pencil (or a sharp lead pencil), and a

couple of tooth-picks.

spot Map.

Cool the pencil in the ice.

water. Dry it and pass it over the skin. Whenever a cold spot flashes out, mark its position in blue ink with a tooth-pick on the cast. The fine creases in the skin will enable you to locate it exactly. Repeat this a few times, till you are satisfied that you have a map of all the cold spots. Warm the pencil in the hot water and find the hot spots in the same way. Mark them on the cast in red ink. When you have finished you will have a complete geography of your temperature spots on a relief map.

FIG. 71.-Finding the Hot and Cold Spots.

The hot spots are ordinarily not sensitive to coldness or the cold spots to heat. Yet a very hot point applied to a cold spot so as not to reach hot spots also will feel cold; of course, to a hot spot it is intensely

hot. It is noteworthy that when the hand is applied to a very hot or a very cold object there is often doubt for a few moments whether it is hot or cold, because both sets of spots are stimulated.

The temperature spots answer to tapping by sensations of hot or cold. For an experiment, choose a sensitive cold spot and let some one tap it with a fine wooden point, it will feel cold. Thrust a needle into it; it will feel no pain.

In studying the subject of touch we had occasion to notice a certain law of change (page 79). Does such a law hold good for hot and cold? By experiments with a special apparatus I was able to prove that it did; the smallest noticeable change depended on the rate of change. But complicated apparatus is not necessary to illustrate the law; anybody can do it by means of a lamp and a spoon. Let some one else hold the spoon by the extreme end; you yourself put your finger about half way down the handle. The bowl of the spoon is now held over a lamp so that it will slowly become hot. After a while the handle of the spoon under your finger begins to feel slightly warm. Lift the finger and immediately place the same finger of the other hand on the same place. The spoon will be found to be quite warm or even painfully hot. When the heat was gradually increased it was scarcely noticed, but when suddenly increased, it was clear at once; in short, the sensitiveness to heat depends on the rate of change.

Although a frog jumps readily when put in warm water, yet a frog can be boiled without a movement, if the water is heated slowly enough. In one experi