PHARMACY. 



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 



VOL. V. 



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 via hnmidd, 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 via 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- 

 struum. 



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. 



Particular terms have been applied to 

 particular cases of solution. 



The solution of a fluid in the atmos- 

 phere is termed spontaneous evaporation. 

 It is promoted by exposing a large sur- 

 face, by frequently renewing the air in 

 contact with the surface, and by increase 

 of temperature. 



Some solids have so strong an affinity 

 for water, that they attract it from the at- 

 mosphere in sufficient quantity to dissolve 

 them ; these are said to deliquesce ; others, 

 on the contrarv, retain their water of crys- 

 tallization with so weak a force, that the 

 atmosphere attracts it from them, so that 

 they crumble into powder. These are 

 said to effloresce. Both operations are 

 promoted by exposing large surfaces, and 

 by a current of air; but the latter is facili- 

 tated by a warm, dry air, and the former 

 by a cold, humid atmosphere. 



Solution is also employed to separate 

 substances, (for example, saline bodies), 

 which are soluble in the menstruum, 

 from others which are not. When oav 



Cc 



