We have seen in an earlier experiment that twice as much hydrogen as oxygen can be obtained from water. Two atoms of the element hydrogen unite with one atom of the element oxygen to make one molecule of water. In symbols we express this H_2O. A group of symbols, such as this, expressing a molecule of a compound is called a _formula_. NaCl is the formula for sodium chloride, which is the chemical name of common salt. CHAPTER X LIGHT 98. What Light Does for Us. Heat keeps us warm, cooks our food, drives our engines, and in a thousand ways makes life comfortable and pleasant, but what should we do without light? How many of us could be happy even though warm and well fed if we were forced to live in the dark where the sunbeams never flickered, where the shadows never stole across the floor, and where the soft twilight could not tell us that the day was done? Heat and light are the two most important physical factors in life; we cannot say which is the more necessary, because in the extreme cold or arctic regions man cannot live, and in the dark places where the light never penetrates man sickens and dies. Both heat and light are essential to life, and each has its own part to play in the varied existence of man and plant and animal. Light enables us to see the world around us, makes the beautiful colors of the trees and flowers, enables us to read, is essential to the taking of photographs, gives us our moving pictures and our magic lanterns, produces the exquisite tints of stained-glass windows, and brings us the joy of the rainbow. We do not always realize that light is beneficial, because sometimes it fades our clothing and our carpets, and burns our skin and makes it sore. But we shall see that even these apparently harmful effects of light are in reality of great value in man's constant battle against disease.