[What are the first causes of loss of manure? What is _evaporation_?] The loss of manure is a subject which demands most serious attention. Until within a few years, little was known about the true character of manures, and consequently, of the importance of protecting them against loss. The first causes of waste are _evaporation_ and _leaching_. EVAPORATION. [Name a solid body which evaporates. What takes place when a dead animal is exposed to the atmosphere for a sufficient time? What often assist the evaporation of solids?] Evaporation is the changing of a solid or liquid body to a vapory form. Thus common smelling salts, a solid, if left exposed, passes into the atmosphere in the form of a gas or vapor. Water, a liquid, evaporates, and becomes a vapor in the atmosphere. This is the case with very many substances, and in organic nature, both solid and liquid, they are liable to assume a gaseous form, and become mixed with the atmosphere. They are not destroyed, but are merely changed in form. As an instance of this action, suppose an animal to die and to decay on the surface of the earth. After a time, the flesh will entirely disappear, but is not lost. It no longer exists as the flesh of an animal, but its carbon, hydrogen, oxygen, and nitrogen, still exist in the air. They have been liberated from the attractions which held them together, and have passed away; but (as we already know from what has been said in a former section) they are ready to be again taken up by plants, and pressed into the service of life. The evaporation of liquids may take place without the aid of any thing but heat; still, in the case of solids, it is often assisted by decay and combustion, which break up the bonds that hold the constituents of bodies together, and thus enable them to return to the atmosphere, from which they were originally derived. [What is the cause of odor? When we perceive an odor, what is taking place? Why do manures give off offensive odors? How may we detect ammonia escaping from manure?] It must be recollected that every thing, which has an _odor_ (or can be smelled), is evaporating. The odor is caused by parts of the body floating in the air, and acting on the nerves of the nose. This is an invariable rule; and, when we perceive an odor, we may be sure that parts of the material, from which it emanates, are escaping. If we perceive the odor of an apple, it is because parts of the volatile oils of the apple enter the nose. The same is true when we smell hartshorn, cologne, etc. Manures made by animals have an offensive odor, simply because volatile parts of the manure escape into the air, and are therefore made perceptible. All organic parts in turn become volatile, assuming a gaseous form as they decompose.