222 THE POPULAR SCIENCE MONTHLY 



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 moun- 

 tains 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 curi- 

 osity 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 ma- 

 king 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 crys- 

 tallization. 



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 im- 

 portant 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, 



