It will be noticed that at the bottom of the circle represented in Fig. 25, at A, are given various ingredients which are found in the soil and which form plant foods.  Plant foods, as may be seen there, are obtained partly from the air as carbonic dioxide and water; but another portion comes from the soil.  Among the soil ingredients the most prominent are nitrates, which are the forms of nitrogen compounds most easily made use of by plants as a source of this important element.  It should be stated also that there are other compounds in the soil which furnish plants with part of their food—­compounds containing potassium, phosphorus, and some other elements.  For simplicity’s sake, however, these will be left out of consideration.  Beginning at the bottom of the cycle (Fig. 25 A), plant life seizes the gases from the air and these foods from the soil, and by means of the energy furnished it by the sun’s rays builds these simple chemical compounds into more complex ones.  This gives us the second step, as shown in Fig. 25 B, the products of plant life.  These products of plant life consist of such materials as sugar, starches, fats, and proteids, all of which have been manufactured by the plant from the ingredients furnished it from the soil and air, and through the agency of the sun’s rays.  These products of plant life now form foods for the animal kingdom.  Starches, fats, and proteids are animal foods, and upon such complex bodies alone can the animal kingdom be fed.  Animal life, standing high up in the circle, is not capable of extracting its nutriment from the soil, but must take the more complex foods which have been manufactured by plant life.  These complex foods enter now into the animal and take their place in the animal body.  By the animal activities, some of the foods are at once decomposed into carbonic acid and water, which, being dissipated into the air, are brought back at once into the condition in which they can serve again as plant food.  This part of the food is thus brought back again to the bottom of the circle (Fig. 25, dotted lines).  But while it is true that animals do thus reduce some of their foods to the simple condition of carbonic acid and water, this is not true of most of the foods which contain nitrogen.  The nitrogenous foods are as necessary for the life as the carbon foods, and animals do not reduce their nitrogenous foods to the condition in which plants can prey upon them.  While plants furnish them with nitrogenous food, they can not give it back to the plants.  Part of the nitrogenous foods animals build into new albumins (Fig. 25 C); but a part of them they reduce at once into a somewhat simpler condition known as urea.  Urea is the form in which the nitrogen is commonly excreted from the animal body.  But urea is not a plant food; for ordinary plants are entirely unable to make use of it.  Part of the nitrogen eaten by the animal is stored up in its body, and thus the body of the animal, after it has died, contains these nitrogen