Congelation is the reduction of a fluid to a solid form, in consequence of the ab straction of caloric. The menus employ ed for abstracting the caloric, are the evaporation of volatile fluids, the solution of solids, and the contact of cold bodies.
Coagulation is the conversion of a fluid into a solid of greater or less consistence, merely in consequence of a new arrange ment of its particles, as during' the pro cess there is no separation of calorie or any other substance. The means of pro ducing coagulation are, increase of tem perature, and the addition of certain substances, as acids and rennets.
Chemical Combination, is the intimate union of the particles of at least two he terogeneous bodies. It is the effect re sulting from the exertion of the attraction of affinity, and is therefore subjected to all the laws of affinity.
To produce the chemical union of any two or more bodies, it is necessary, that they possess an affinity for each other ; that their particles come into actual con tact ; that the strength of the affinity be greater than any counteracting causes which may be present.
The principal counteracting causes are, the attraction of aggregation ; and affini ties for other substances. The means to be employed for overcoming the action of other affinities, will be treated of un der Decomposition. The attraction of aggregation is overcome by means of mechanical division ; or the action of ca loric.
Combination is facilitated by increasing the points of actual contact, by the means of mechanical agitation ; by condensation and compression ; and the processes em ployed for producing combination may be considered, with regard to the nature of the substances combined, and to the nature of the compound produced. Gases combine with gases, and dissolve fluids or solids, or are absorbed by them. Fluids are dissolved in gases, or absorb them : they combine with fluids, and dissolve solids, or are rendered solid by them. Solids are dissolved in fluids and in gases, or absorb gases, and solidify fluids.
The combination of gases with each other, in some instances, takes place when simply mixed together : thus ni trous and oxygen gases combine as soon as they come into contact ; in other in stances, it is necessary to elevate their temperature to a degree sufficient for their inflammation, either by means of the electric spark, or the contact of an ignited body, as in the combination of oxygen gas with hydrogen or nitrogen gas.
When gases combine with each other there is always a considerable diminution of bulk, and not unfrequently they are condensed into a liquid or solid form. Hy drogen and oxygen gases form water; mu riatic acid and ammonia gases form solid muriate of ammonia. But when the com bination is effected by ignition, a violent expansion, which endangers the bursting of the vessels,previously takes place, in consequence of the increase of tempera ture.
Solution is the diminution of aggrega tion in any solid or fluid substance, in con sequence of its entering into chemical combination. The substance. whether solid or fluid, whose aggregation is les sened, is termed the solvend ; and the substance, by whose agency the solution is effected, is often called the menstruum or solvent. Solution is said to be per formed aid humidd, when the natural form of the solvent is fluid; but when the agen cy of heat is necessary to give the solvent its fluid form, the solution is said to be performed aid siccd. The dissolving pow er of each menstruum is limited, and is determinate with regard to each solvend. The solubility of bodies is also limited and determinate with regard to each men struu m.
When any menstruum has dissolved the greatest possible quantity of any sol vend, it is said to be saturated with it. But, in some cases, although saturated with one substance, it is still capable of dissolving others. Thus a saturated so lution of muriate of soda will dissolve a certain quantity of nitrate of potash, and after that, a portion of muriate of ammo nia.
The dissolving power of solvents, and consequently the solubility of solvends, are generally increased by increase of temperature : and conversely, this power is diminished by diminution of tempera ture; so that from a saturated solution, a separation of a portion of the solvend ge nerally takes place on any reduction of temperature. This property becomes extremely useful in many chemical ope rations, especially in crystallization.