Chemical Affinity, the name given to the force which combines together chemical elements so as to form compounds. Of its real nature or essence we are entirely ignorant, as we are of the essential nature of other material forces. The term chemical attraction has also been applied to this force, on the hypothesis that it draws together chemical atoms. In many cases there can be no doubt that the chemical particles come nearer together when they combine: thus if two volumes of hydrogen and one volume of oxygen be caused to unite, we do not get three volumes of steam, but only two; that is, the particles have ap-proached so much closer in combining as to occupy but two thirds of their former space. In other cases, however, compounds are found to occupy exactly the same space that their elements did before combination, and sometimes they fill even a greater space. Hence the term chemical attraction has been thought objectionable. Chemical affinity is that link or tie which binds together unlike kinds of matter, in such an intimate manner that the properties of the elements are lost, and a compound with new properties is produced. It is in this that it differs from cohesion, which only unites or aggregates similar particles without altering properties.

The particles in a piece of iron or sulphur are held in union by cohesion; but when sulphur and iron combine chemically, both elements disappear, lose their properties and identity, and a new compound is formed - the sulphuret of iron. Newness of properties in the compounds formed is the distinguishing peculiarity i of chemical affinity. It obliterates the characteristics of the elements, and generates new properties in the product. Cohesion is usually said to act between homogeneous particles, as in the cases just cited of sulphur and iron; but it may also act between dissimilar substances, as where silver is inlaid with steel, or copper metal united to tin, or iron coated with zinc, or wood joined to glue, or paper to paste, or pitch to the fingers. These, however, are mechanical combinations; there is no destruction of the properties of the combined substances, and those of the combination are not new, but are the same as the properties of the constituent substances, each of which retains its individuality. The force of gravitation is brought into play between masses of matter at all distances; chemical affinity acts only when the elements are in contact or at insensible distances.

For this reason affinity is most energetic when one or both of the elements are in a state of solution, the approach of the atoms being then most perfect. It was once thought that chemical affinity could not take effect without the intervention of solution; and although the statement is generally true, yet there are some substances whose affinities are so intense that they will unite even in the solid state when made to touch each other. The action of affinity is heightened, modified, and suspended by various other causes. Among these heat is most potent, and most easily available in the laboratory and chemical manufactory. Thus carbonic acid and lime unite strongly at common temperatures, forming marble or limestone, but at a red heat their affinity is annihi-lated and they separate. On the other hand, potash and sand will not actively combine at ordinary temperatures, while at a red or white heat, at which they are melted, combination takes place and glass is formed. Light also influences affinity, promoting combination and decomposition. If chlorine and hydrogen gases be mixed in the dark they will not unite, but exposed to light they combine at once; while in every green vegetable leaf carbonic acid is decomposed every day under the inthi-ence of solar light.

The recent investigations in photography have greatly multiplied the number of substances over which light is known to exert a chemical influence. Electricity also has a governing action over affinity. An electric spark, shot through a mixture of oxygen and hydrogen gases, causes them to combine instantaneously and explosively, producing water; while a steady electric stream sent through the water annuls the affinity of its elements and sets them free again. Other causes also, known and unknown, affect in various ways and degrees the play of affinity; indeed, a full statement of them would involve almost the whole science of chemistry. - The changes in the properties of substances produced by affinity are numberless and surprising. When solid charcoal and sulphur combine, the compound formed is colorless as water, and highly volatile. If yellow sulphur and bluish white quicksilver be heated together, they form the bright red vermilion. Waxy phosphorus and colorless invisible oxygen unite to form a white body resembling snow. Nitrogen and oxygen are tasteless, separate or mixed; yet one of their compounds, laughing gas, is sweet, and another, nitric acid, intensely sour; they are both transparent and invisible, yet they form a cherry-red compound gas.

Charcoal and hydrogen are odorless; nevertheless, many of our choicest perfumes, such as oils of roses and bergamot, as well as the less agreeable spirits of turpentine and illuminating gas, contain only these elements. The mild and scentless nitrogen and hydrogen give rise to one of the most odorous and pungent compounds, ammonia; while suffocating and poisonous chlorine, united to a bright metal, sodium, yields common salt. Charcoal, hydrogen, and nitrogen, which singly or mixed are not injurious to life, yet combine to form the terrible poison prussic acid; while charcoal, hydrogen, and oxygen, variously united, produce sweet sugar, poisonous oxalic acid, and intoxicating alcohol.- The strength of affinity among different elements is various. Thus the chemical energies of sulphuric acid are superior to those of carbonic acid; if the former be united to carbonate of lime, it takes the lime away from the carbonic acid - that is, produces decomposition and a new compound. It has been attempted to establish a scale of affinities among various chemical substances to form the basis of an order of decomposition; but affinity is disturbed and overcome by so many circumstances that such tables are of but little value.

For the laws of affinity or chemical combination, see Atomic Theory.