As soon as the beat is applied, the spirit being more easily vapourized, rises to the top of the retort, where, being condensed into the liquid state, it slowly trickles down the neck into the receiver. The neck of the retort and the receiver may be kept cool, by applying cloths, constantly moistened with cold water, to their external surface. The spirit may also be separated from the water by intense cold, which freezes the water, and leaves the spirit in its liquid state. Mixtures of gaseous fluids are not separable by heat or cold, for they all expand equally by equal additions of heat ; but steam, and other vapours, may be separated from gases by reduction of temperature, which destroys the elasticity of the former, but not of the latter. The second degree of chemical attraction is solution, which operates between solids and liquids, or gases and liquids: the liquids, in these cases, are called solvents. Between some liquids and solids attraction does not exist, or it is overbalanced by the cohesive power. Resin and sealing wax are insoluble in water, but readily dissolve in alcohol. Chemical attraction may be exerted in different degrees between one body and several others. There is an affinity between alcohol and water, whereby they are capable of mixing; there is also a mutual affinity between alcohol and resin, by which the former dissolves the latter ; but there is no affinity between water and resin. Now, if water be added to a solution of resin in alcohol, the resin will resume tie solid form ; the attraction between the particles of alcohol and those of water is greater than that between the particles of resin and alcohol : the consequence is, that the alcohol quits the resin and combines with the water. and the resin falls to the bottom of the liquid. This is called elective attraction, because the alcohol may be considered as exercising a choice between the substances with which it is capable of combining. This resumption of the solid form by a body previously dissolved in a liquid, is termed precipitation. The power of solution is limited, as liquids cannot combine with more than a certain definite quantity of any solid or aeriform body. The point at which the action between the two bodies ceases is called the point of saturation. Up to this point the two bodies may combine in any proportions. A solvent that has been saturated with one substance, is often capable of combining at the same time with others ; thus water that is saturated with saltpetre will dissolve a considerable quantity of common salt. The influence of heat upon the power of solution corresponds with the difference between cohesion and elasticity. Upon solid bodies it generally increases the power of the solvent by diminishing their cohesion. Upon aeriform bodies it diminishes thepower by increasing their elasticity. Solvents may be separated from the bodies with which they are united by alterations of temperature. If a solution of alum or common salt be boiled, the liquid will be dissipated in vapour, but the solid salt will remain. If a solution of carbonic acid in water be frozen, the gas will escape and the water remain in the solid state. The result of the highest degree of chemical attraction is composition, which may take place between bodies under every modification of cohesive attraction. Bodies that unite in this intimate manner combine only in definite proportions, which are invariable in the same compound. Combination usually produces a total change in the sensible pro perties of the combining substances; and the process is generally accompanied by change of temperature, to such an extent sometimes that light and hest are emitted in abundance. If copper filings and sulphur be mixed together, and heat applied as soon as the sulphur melts, a violent action will take place, the copper will become red hot, and a black brittle body will be formed, with properties totally different from those of its two ingredients. This compound, which is the sulphuret of copper, is often found ready formed in mineral veins; but whether existing naturally, or formed by art, its composition is definite and invariable. It is found to consist of 64 parts by weight of copper, and 16 of sulphur. There is, however, another compound of copper and sulphur, in which the proportions are 64 copper to 32 of sulphur. In this it will be seen that the quantity of sulphur is exactly double of that which the first combi nation contains. This is a fact of common occurrence; many bodies will unite in two or three different proportions, but they are no less definite, each being a multiple, or a submultiple, of the preceding; that is, it is double, triple, or half, and so on. This is well illustrated in the compounds of mercury. If this brilliant and fluid metal be agitated for a long time in contact with the common air, it will unite with the oxygen of that mixture, and will be con verted into a black, insipid, insoluble powder, which consists of 200 parts of mercury to 8 of oxygen. If instead of being agitated at common temperatures with the air it be kept nearly boiling in it, it will be converted into a red shining mass, endued with an acrid metallic taste, and which is soluble in water, and poisonous. This consists of 200 parts of mercury, and 16 of oxygen. Again, if mercury be rubbed in a mortar with sulphur, it unites with a given proportion. If the mercury be poured into melted sulphur, and strongly heated, a compound will be formed which rises in vapour, and concretes on cooling into a brilliant red substance, known as cinnabar or vermilion. In this the quantity of sulphur is double of that in the preceding compound. Another compound of mercury is the well-known medicine called calomel, which is formed by the union of the metal with a gas named chlorine. The compound is heavy, white, tasteless, crystalline, and nearly insoluble in water. It may be taken in doses of several grains without any effect but that of a purgative. It is composed of 200 parts of mercury, and 36 of chlorine. If, however, the mercury be made to combine with 72 parts of chlorine, it produces corrosive sublimate, a semitransparent white mass, of an acrid, nau seous, metallic taste, soluble in water and alcohol, and highly poisonous. In these examples, it will be seen that substances comparatively inert, may produce by their union compounds of highly active properties ; and that a compound of two bodies, which, in one proportion, if taken internally, will have but a slight effect upon the animal frame, if united in a different proportion will prove destructive to life. On the other hand, highly active bodies of opposite properties will also produce, by their union, substances of a mild cha racter, and they are then said to neutralise each other. Sulphuric acid, or oil of vitriol, is highly corrosive, and intensely sour. If brought into contact with any substance stained with vegetable blue colours, as those of violets or litmus, it instantly changes them to a bright red, a property which belongs to all the acids. Barilla, or carbonate of soda, is a solid that is soluble in water, of a hot, acrid, bitter taste. It changes the blue colour of vegetables to green, which property is common to all alkalies. If into a solution of this substance we carefully drop a portion of the former, a brisk effervescence will ensue, and carbonic acid will be riven off. If the experiment be performed with care, and the afFusion of the acid stopped the moment the effervescence ceases, the solu tion will be found to be warm, and possessed of properties totally different from those of the acid or alkali from which they have been formed; it will no longer be sour, corrosive, acrid, or hot ; it will have no action on vegetable colours, and all the active properties of the original bodies are said to be neu tralised. The compound is slightly bitter, saline, and cooling. If part of the water be evaporated by heat, a solid will be deposited in regular crystalline forms, called sulphate of soda, or Glauber's salt. The operation of affinity, which produces chemical composition, exists in a body in different degrees towards other bodies ; and if to a compound of two bodies a third be pre sented which has a stronger attraction for either of the component parts than they have for each other, a decomposition will take place, and a new compound will result. Carbonate of soda is composed of carbonic acid and soda ; and in the experiment above referred to, it is decomposed, the sulphuric acid uniting with the soda, and the carbonic acid escaping in the gaseous state. In like manner, if a solution of carbonate of soda be boiled with caustic lime, the car bonate of soda will be decomposed, the carbonic acid will quit the soda and unite with the lime, forming carbonate of lime or chalk, and caustic soda will remain in the solution. These affinities are conveniently represented in the following diagrams :— In these diagrams the substance at bottom is the new compound, which is cipitated, or thrown down, and that at the top either escapes or remains in solution ; this action is called single elective affinity. If two compounds are brought together in solution, it not unfrequently happens that a process of double decomposition and composition occurs ; that is, the two original bodies are decomposed, and two new compounds produced, by a mutual interchange of ingredients : this compound action has been named double elective ((fixity. Decomposition, which cannot be effected by single, may by double elective affinity. Sugar of lead, or acetate of lead, is a compound of acetic acid and lead. White vitriol, or sulphate of zinc, is compounded of sulphuric acid and zinc. If in a solution of acetate of lead we suspend a piece of metallic zinc, we shall have an example of single elective affinity ; the acetic acid will act upon the zinc, and the lead will resume its metallic state, and be precipitated on the zinc in a beautiful arborescent form. But if a solution of acetate of lead be mixed with a solution of sulphate of zinc, the acetic acid will unite with the zinc, and, at the same instant, the sulphuric acid will combine with the lead, forming an insoluble precipitate, which will fall to the bottom of the vessel while the acetate of zinc remains in solution. The latter is an instance
of double elective affinity, and may be represented by the following diagram :— Here the original substances are placed on the right and left of the diagram, while the nevi compounds occupy the top and bottom.
Having given a general view of the nature of chemical attraction, we shall next proceed to sketch the most important facts as connected with this subject relative to light, heat, and electricity. The phenomena resulting from the motion of light will be found in the article Orrice; we shall, therefore, in the present sketch, refer to the mechanical properties of light only so far as is necessary to render its chemical effects intelligible. When a pencil of light is admitted through a hole in a window shutter into a darkened room, and made to fall on a triangular glass prism, it is turned out of its natural or straight-lined direction, and prevented from falling on the floor, where it would produce a spot of white light, instead of which it forms a spectrum of splendid colours on the wall, or on a screen placed to receive it. From this circumstance it appears that light is compound and separable by means of an inequality in the refrangibility. In the annexed diagram a b represents a ray of light, which falling on the prism e, is diverted from its course, and dispersed into colours occupying the space c d. The colours, proceeding from the bottom, are red, orange, yellow, green, blue, indigo, and violet, according to Sir Isaac Newton's observation; but some modern observers state that there are only red, green, blue, and violet, in the spectrum, when it is formed as it should be, witii a very small pencil of light. The violet occupying the upper part of the spectrum, is most diverted from its course, and is said, therefore, to be the most refrangible. The red is the least refrangible. This effect is very similar to that of elective attraction, for the same glass acts with different force on different rays ; and this analogy is extended by the observation that different kinds of glass, as well as other substances, disperse light in different proportions. If these differently coloured rays of light thus separated by the prism be concentrated on one spot by means of a lens, they will reproduce colourless light. If the spectrum produced, as we have stated, be minutely examined, it will be found to have different pro perties in different parts. Thus the red end will most sensibly erect the thermometer; the lightest green rays are most illuminative, and the violet end produces the most decided chemical changes. If the white lane cornea, the muriate of silver, be moistened and exposed to the different rays in the pris matic spectrum, it will be found that no effect is produced upon it in the least refrangible rays, which occasion heat without light. A slight discolouratiou will be occasioned by the red rays, but the blackening power will be greater in the violet than in any other ray ; and beyond the violet, in a space perfectly dark, the effect was still perceptible. This observation shows that there are rays more refrangible than those which produce heat and light. Sir H. Davy found that a mixture of chlorine and hydrogen acted more rapidly upon each other, combining without explosion, when exposed to the red, than when placed in the violet rays ; but that solution of chlorine in water became solution of muriatic acid most rapidly when placed in the most refrangible rays of the spectrum. He also observed that the puce-coloured oxide of lead, when mois tened, gradually gained a tint of red in the least refrangible rays, and at last became black, but was not affected in the most refrangible. The same change was produced by exposing it to a current of hydrogen gas.
Dr. Wollaston found that gum; exposed to the violet rays, passed rapidly from yellow to green. MM. Gay Lussac and Thenard applied the same influence to a gaseous mixture of hydrogen and chlorine, when an explosion immediately took place. By placing small bits of card, coated with moist horn silver, or little phials of those mixed gases, in different parts of the spectrum, M. Berard verified the former observations, of the chemical power acquiring its maximum in the violet ray, and existing even beyond it; but he also found by leaving the tests a sufficient time in the indigo and blue rays, a perceptible effect was produced by them also. He concentrated by a lens all that portion of the spectrum which extends from the green to the extreme boundary of the violet, and by another lens he collected the other half of the spectrum, com prehending the red. The latter formed a focus of white light so brilliant that the eye could not endure it, yet in two hours it produced no sensible effect on muriate of silver. On the contrary, the focus of the other half of the spectrum, whose light and heat were far less intense, blackened the muriate in ten minutes. The sun beams, in traversing a produce similar effects. Thus, the chloride of silver acquires a black tint behind a blue or violet glass, but does not blacken behind a red or orange glass. On the other hand, it becomes red behind a red glass, and that much more quickly than even in the solar spectrum. With respect to the light emitted by gases, even the bright light of olefiant gas, when concentrated so as to produce a sensible degree of heat, occasioned no change on the colour of muriate of silver, nor on a mixture of chlorine and hydrogen while the light emitted by elect:1=d charcoal speedily affects the muriate, and causes these gases to unite rapidly, and sometimes with explosion. Sit H. Davy has remarked, that the refraction and effects of the solar beam offer an analogy to the agencies of electricity. In the voltaic circuit, the maximum of heat seems to be at the positive pole, where the power of combining with oxygen is given to bodies, and the agency of rendering inflammable is exerted at the opposite surface ; and similar chemical effects are produced by negative electricity, and by the most refrangible rays of the solar beam. In general, in nature, the effects of the solar rays are very compounded. Healthy vegetation depends upon the presence of the solar beams, or of light; and whilst the heat gives fluidity and nobility to the vegetable juices, chemical effects likewise are occasioned—oxygen is separated from them and inflammable compounds formed. Plants deprived of light become white, and contain an excess of saccharine and aqueous particles, and flowers owe the variety of their hues to the influence of the solar beams. Even animals require the presence of the rays of the sun, and their colours seem materially to depend upon the chemical influence of these rays; a comparison between the polar and tropical animals, and between the parts of their bodies exposed, and those not exposed to light, shows the correctness of this opinion. Light is produced in several natural operations, most of which may be con sidered in this place. If two pieces of quartz be rubbed together, light is produced even under water. Atmospheric air, or oxygen, quickly and violently lompressed in a glass syringe, or a glass ball filled with the latter, and suddenly broken in vacuo, produces light. Light accompanies intense heat. Air heated up to 900° Fahr. and made to fall on pieces of metal, earth, &c. communicates to them the power of radiating light. The flame exhibited in the burning of charcoal and phosphorus is merely the ignition of the solid particles of these bodies. At a certain elevation of temperature, about 800° of Fahr., all solid bodies begin to give out light, and the same effect is produced in vacuo by transmitting voltaic electricity through a metallic wire. The phosphorescence of minerals is another source of light. If fluor spar be coarsely pounded and placed upon a mass of iron heated below redness, it will give out a beautiful green light; this property is possessed by quartz, topaz, phosphate of lime, and a variety of other minerals. There is also a class of bodies called solar phosphoric, which emit light upon exposure to any highly luminous body; the moat powerful of these is a compound discovered by Canton. If three parts of calcined oyster shells in powder are mixed with one of flour of sulphur, and the mixture rammed into a crucible, and ignited for half an hour, we shall find that the bright parts, on exposure to the sun-beam, or to the common daylight, or to an electric explosion, will acquire the faculty of shining in the dark so as to render visible the figures on the dial-plate of a watch. After a while they will cease to shine ; but if the powder be kept in a well corked phial, a new exposure to the sun-beam will restore the luminescence. When an electric discharge is transmitted along the surfaces of certain bodies, a somewhat durable phosphorescence is occasioned ; thus— Canton's pyrophorous produces more light by this treatment than any other body, but nearly every native mineral, except metallic ores and metals, become luminous by an electric explosion. Light is also emitted in certain chemical changes, where no sensible heat is perceived. Marine animals, both living and dead, emit light ; as the shell fish called pholas, the medusa phosphorea, and other molluscan. When deprived of life, marine fishes in general seem to abound in this kind of light; among insects, also, several species of/edgers, or lantern fly, and of lampyris, or glow-worm. Rotten wood and peat earth also emit light copiously. This luminous matter may be separated from the bodies of animals by immersing them for a short time in dilute saline solutions ; one part of sulphate of magnesia in eight of water is the most convenient menstruum for this purpose. Fresh water, spirit of wine, and dilute acids, destroy the luminous property altogether, while a gentle heat, and oxygen gas, increase the brightness of the phosphorescence.