This is known as a _structural_ or _constitutional_ formula, and from it we can see at a glance that A is bivalent, C is quadrivalent, and B univalent. This type of formula shows us more readily than the other the structure of the molecule in question. The complexity of such formulæ naturally increases with the complexity of the molecules, and in many cases may be extremely intricate. Ordinary chemical formulæ, however, are written empirically. Any chemical changes which take place as the result of some reaction are expressed in this manner. ORGANIC VS. INORGANIC CHEMISTRY As before stated, chemistry has been divided into two categories--organic and inorganic. It was stated at the time that these divisions represented the chemistry of living and dead matter, respectively. As a matter-of-fact, this description is not quite accurate. This was the older view of the observed facts, because it was believed that some mysterious “vitality” was responsible for the peculiar substances found in living bodies, but the chemist has now succeeded in making, in the laboratory, a number of these substances which were thought to be the result of life only; and in addition has succeeded in making great numbers of organic compounds not found in the living body. Over 150,000 “organic” compounds are now known to the chemist, only a small fraction of which are known to be the product of “vitality.” All living things--animal and vegetable--contain carbon, as their most important constituent, so that the modern view of organic chemistry is that it is, very largely, the chemistry of carbon compounds. Whether or not any form of “vitality” exists aside from the living matter studied is a question usually passed over by chemists as beyond their province. There is no doubt, however, that the human body presents many problems still unexplained by modern chemistry. Take, for example, the miracle of digestion. A potato, a cabbage, an apple, a chicken running about the yard, a piece of candy--all these are eaten by little Mary Jones, and are somehow turned into the body of little Mary Jones, making hair, teeth, eyes, lungs, liver, nerves, brain, etc. The food material is somehow transformed into the living body of the person eating it! Much has been discovered as to the innumerable changes which the food undergoes during the various stages of digestion, but the final result--how this pabulum is converted into bodily tissue--is still largely a mystery. We know, for example, that proteins are broken-up into simpler compounds, the most important of which are the amino-acids. Fats are broken up into fatty acids and glycerine, and substances resembling soaps are formed in the body. Carbohydrates are resolved into levulose, glucose, maltose, etc., which are utilizable by the human system. But just how these substances are converted into bodily tissue is still largely a problem. ORGANIC COMPOUNDS