Page:Popular Science Monthly Volume 84.djvu/226

222 is arbitrarily assigned to it under these conditions, and all other values of temperature are referred to this and to the freezing point, 0° C, as standards. If the pressure is changed, the raising or lowering of the boiling point is directly in proportion.

Egg albumen coagulates only very slowly at temperatures below 100° C, and since the atmospheric pressure on the top of high mountains is quite a bit lower than at their foot, we see from the above why an egg takes so very much longer to cook at such elevations, if it cooks at all.

Water is a powerful refractor of light. This can be best shown by holding a stick in it in a slanting position, so that part of it protrudes above the surface. The stick appears to be bent. An interesting curiosity which makes use of this principle is the fish-eye camera, which makes things in front of it appear just as they would to a fish under water, that is, instead of a limited view of the scenery, or whatever it may be, everything within a radius of 180° is shown in the picture. The camera is a box filled with water; in the back is placed the plate, and the light enters through a small hole in the front.

Most substances, when dissolved in water, lower its freezing point. That is one reason why salt is used in the freezing mixture when making ice cream, the temperature of the ice salt mixture surrounding the can in a "freezer" often reaching a temperature of—21° C. In this connection it may be said that the stirring which is carried on serves two purposes; it brings the entire contents of the can into contact with the cold walls of it, which radiate the heat very rapidly to the outside; it likewise causes a more rapid crystallization of the contents, and in consequence makes the crystals much smaller.

Mention should also be made here of the undercooling which takes place when a solution is cooled. Instead of ice forming at the freezing temperature of the solution, by keeping it quiet and out of contact with the air, the solution will remain in the liquid state several degrees below that point. A small crystal of the solvent, or a sharp-edged body, or even a jar, will cause it to freeze suddenly.

Similar to this is a supersaturated solution, or one in which more of the substance is dissolved than it can ordinarily hold, a crystal of the dissolved substance, or the other treatments spoken of, causing crystallization.

Besides lowering the freezing point, dissolving a substance in a liquid also raises the boiling point. Much could be written concerning both phenomena but space does not permit. It is enough to say that the relationships established by a study of them are some of the most important of all science. Of course every substance has its own effect and the amount of each dissolved, has to be taken into account as well.

When an acid, base or salt is dissolved in water, it is dissociated,