300 



CHEMISTIIY. 



[CHEMICAL ATTKACTIOW. 



CHAPTER I. 

 CIIKMICAL ATTRACTION, OR AFFINITY. 



\V hsvp h i.| fr.-mont occasion, in our previous page*, 

 t,> attraction of various kind*. Tho attrao- 



.f gravitation we found to subsist between masses; 

 that of cohesion between the particloa of Ixnlios, ope- 

 ratir; :uible distances ; whilst the attractions 



.t.vl by free electricity and magnetism, were 

 special or individual in their character. But, in che- 

 mical attraction, we find an entirely different class of 

 phenomena to occur when compared with those we have 

 .; , . ;...;. Hi- <;... i! li.i< ii"! !> n of miK-li DOBr 

 sequence a to the qualities or specific character of the 

 substance* we have experimented with ; it will now bo 

 our business to investigate those points as closely as 



By chemical attraction, we mean that power by virtue of 

 which two or more dissimilar bodies are brought together, 

 united, and ultimately produce another body totally 

 different, in most respects, to those from which it was 

 derived. One of the simplest instances of this kind 

 which we can adduce, is that of common salt It is 

 composed of two substances only ; one of which, chlorine, 

 is a poisonous gas, that acts with great and destructive 

 effect on animal tissues: the other constituent is a 

 metal called sodium, which spontaneously takes fire on 

 Ivini* brought into contact with water, or paper moist- 

 ened with that liquid. 



We must here point out the difference which exists 

 b tweon a chemically constituted substance ami a mere 

 anieal mixture. Common salt is a compound, in 

 which the parts are held together by so powerful an 

 attraction, that they cannot be divided by any but 

 strictly chemical moans. If, however, we dissolve sugar 

 in water, we do not consider the product to be a che- 

 mical compound, but simply a solution or mixture, 

 because we can readily take the water away from the 

 sugar by the application of heat, and the sugar will be 

 again present, unaltered in any of its characters. In- 

 deed, during the whole process of solution, no change, 

 except a mechanical one, occurs. If, however, we expose 

 sodium to chlorine, a very different result is afforded us : 

 then chemical action ensues ; a combination of the two 

 substances is effected, and a new compound is the result. 



Now, it is by the union of different substances, 

 effected by chemical attraction, that the whole of the 

 materials composing the animal, vegetable, and mineral 

 km.'. loins is composed. The combinations are held to- 

 jMli'-r by varying amounts of force; and the causes 

 which modify the attraction which each body has for 

 those united with it, are modified by several circumstances 

 to which we must now direct attention. 



Amongst all the agents with which we are acquainted, 

 heat is the most powerful in producing mecha- 

 nical and chemical changes ; and to it the chemist is 

 chiefly indebted for the means by which ho is enabled to 

 separate existing compounds, and to form others. Tho 

 mechanical effect* of heat we have already inquired 

 into in our first section, and we shall now more |.ar- 

 ticularly investigate its action on the chemical composition 



So far as we can judge, the mechanical properties 

 ic always precede the development of its chcmic-.il 

 agency; and this will be no matter of surprise if we 

 bear in inin 1, ih it the slightest change from ordinary 

 temperature is sufficient to produce expansion, .tc. Now, 

 chemical action is rarely promoted by caloric, except 

 hen ]. roue-tit in great excess, beyond its ordinary sensible 

 in. Wo may find a very familiar instance of 

 these facts in the ignition of gunpowder. On api>lvin_; 

 heat to that substance, the sulphur is first converted 

 uito a liquid state ; then the nitre ; a red-hoat is next 

 produced; and at that moment a chemical action is 



generated between the charcoal and no <>f the con- 

 stituent* of the saltpetre an explosion immediately 

 ensuing. 



Heat has not only the power of producing such rapid 

 chemical combinations, but it also assists, in many in- 

 etances, to produce those conditions most favourable to 

 suuh results, by altering the molecular construction of 

 bodies. Thus, in making solutions, heat is constantly 

 employed to render bodies more soluble ; and, in many 

 instances, heat produces this result to an astonisliiiu' 

 extent. By such means, a much larger amount of the 

 substance is dissolved than can be held in solution when 

 the liquid again becomes cool ; and the solid substance, 

 on cooling, separates in the form of a powder, or, more 

 generally, it assumes regular forms, and crystallisation 

 ensues. In almost every instance in which we desire 

 to separate bodies from each other, the liquid state is 

 essential for that purpose ; and when solvents cannot 

 conveniently be employed, heat is applied until the body 

 has been brought to the fluid condition. The same 

 plan is adopted when the union of two solid bodies is 

 required. Hence the use of a class of substances called 

 fluxes such as carbonate of soda, borax, and the like. 

 Take, for instance, a piece of common granite, which is 

 composed of several substances. It may be powdered, 

 and then submitted to tho action of every known liquid ; 

 but the chemical attraction of each of its constituents is 

 such as to defy those means for their separation. If, 

 however, after powdering, it be mixed with some car- 

 bonate of soda, and exposed to an intense heat, it is 

 fused, and combines with the flux. It may, when cool, 

 be readily dissolved in acids, <tc. ; and then its con- 

 stituents can be at once separated from each other by 

 the usual methods. 



If, again, it be desired to cause the union of solid 

 bodies, heat readily assists us. Thus, in making an 

 alloy, or mixture of two or more metals, such could 

 never be effected whilst they are in the solid state, 

 because the distance of their particles is too great from 

 each other. But by the aid of intense heat, the partu U * 

 of each arc made mobile ; and although the distances be- 

 tween the atoms of a body are increased, still the readiness 

 with which they are detached from each other, enables 

 them more readily to combine with those of another body ; 

 and thus the state of fusion is favourable to chemical 

 action, as well as to mechanical mixture. The processes 

 of evaporation by the application of heat to liquids, is 

 in common use by the chemist ; and ho thus is enabled to 

 obtain, in a solid form, salts, ttc., from a liquid, although 

 they are present to but a slight extent. Thus, in a 

 gallon of ordinary river water, only two to ten grains of 

 solid matter may be dissolved. By evaporating thu 

 liquid, such are brought together, and are so fit for the 

 experiments on them. 



VVe have previously referred to the effects of beat in 

 producing rapid combination in mixtures of solids : we 

 may now add a few words on the explosion of gases, 

 itc., by the aid of caloric. There are some substances 

 in which tho chemical attraction is so slight, as to be 

 readily dtstroyed even when they are heated to but a 

 trifling extent: amongst these, are fulminating mercury 

 and silver, and curtain compounds of nitrogen witli 

 iodino and chlorine. Now such afford instances in which 

 heat is antagonistic to chemical attraction ; ami many 

 others of a similar kind might be easily adduced. In such 

 cases, gases are always formed, when the substan< 



r a solid or liquid, before the explosion ; and it 

 is owing to tho rapid expansion of these gases, that 

 the noise usually hoard is produced. \Vo shall generally 

 find that the presence of one body, m'trogeu, U mostly 

 to be detected in such instances. 



