rr. 



CHEMICAL AFFINITY. 



CHEMICAL ANALYSIS. 



;to 



uitric acid and sulphate of potaah ; but if a port of the sulphate of 

 potash be dissolved in the nitric acid, nitrate of potaah will be again 

 formed, accompanied with bisulphate of potash. 



There are several other cases which prove that the proportion* of 

 the substances which act chemically upon each uther, greatly influence 

 the nature and proportion* of the new compound* formed : thus, 

 when 100 parts of sulphate of baryta are boiled in a solution of 59 

 part* of carbonate of potaah, 23 of the aulphate are decomposed and 

 converted into 19-3 of carbonate. When also 85 parts of carbonate of 

 baryta are boiled in a solution of 74 of sulphate of potaah, decomposi- 

 tion alao take* place, and there are formed 67 of sulphate of baryta, 

 and 40 of carbonate of potaah. In these experiment*, then, it appears 

 that decomposition cannot in either case be entirely effected, while the 

 new compound* formed remain in mixture with the portion* of the 

 original salt* which remain undecomposed. In other words, there 

 take* place a partition of base* between the acids whose action is 

 opposed to each other. 



Cokauxt, a* the agent tending to produce the solid form of matter, 

 exercises, a* might be expected, a powerful controlling influence upon 

 chemical affinity. It has been already stated, that solids act upon each 

 other chemically with great difficulty, hence cohesion by retaining 

 bodies in the solid form often present* a formidable barrier to their 

 chemical union. Thus pieces of ice and quick-lime may be placed in 

 contact without exhibiting any tendency to combine, although a 

 violent action immediately ensues when an elevation of temperature 

 causes the liquefaction of the ice. The most important and interesting 

 effect* of cohesion upon chemical affinity are, however, observed when 

 the former force comes into play between substance* in solution : thus, 

 when nitrate of baryta is mixed with sulphate of potash, mutual 

 decomposition of the two salts is effected, chiefly by the agency of 

 cohesion, which solidifies and withdraws from solution a compound of 

 the sulphuric acid, of the sulphate of potaah, with the baryta of the 

 nitrate of baryta. This controlling effect of cohesion is one of the most 

 fertile source* of chemical change. 



Heat, according to it* degree and under various circumstances, pro- 

 duces very different effects on chemical affinity, and causes, increases, 

 reverses, or prevent* its action. If we mix oxygen and hydrogen gaaes, 

 they will remain in a state of mixture for an indefinite period without 

 combining ; but if flame be applied to them they combine with explo- 

 sion, and water is formed. When mercury i* moderately heated in 

 atmospheric air it is converted into peroxide, by combining with the 

 oxygen of the air : heat the compound thus formed more strongly than 

 wo* required for it* production, and the affinity is destroyed ; oxygen 

 gas is given out, and the mercury returns to its metallic state. Mix 

 solution* of chloride of calcium and carbonate of ammonia ; double 

 decomposition ensued ; carbonate of lime and chloride of ammonium 

 result Evaporate the mixture to drynees, and heat the residue ; the 

 order of affinities is reversed, and chloride of calcium and carbonate of 

 ammonia are reproduced : in this case, heat reverses the order of 

 atliuities. There is one instance, however, in which heat produces 

 effects that are quite anomalous and irreducible to any idea of their 

 dependence upon degree of temperature ; it i* this : when the vapour 

 of boiling water ia passed over ignited iron, the water is decomposed ; 

 hydrogen, one of it* elements, i* evolved in the state of gas; and 

 oxygen, the other, combine* with the iron and converts it into oxide. 

 Now if we ignite this oxide of iron and pas* hydrogen gas over it, the 

 oxide is decomposed, its oxygen combines with the hydrogen, and 

 water i* re-formed. No satisfactory explanation of these opposing 

 results has hitherto been offered. These fact* afford an illustration of 

 the disturbing causes which very much limit the usefulness of tables 

 of affinity. According to the first experiment, oxygen and iron 

 combine in preference to oxygen and hydrogen, whereas it appears 

 from the second that the affinity of oxygen for hydrogen exceeds that 

 for iron. But it U evident that these statements are irreconcileable 

 with each other ; and as the temperature ia in both cases the same, 

 no variation of it can account for the incompatible results of the 



i remarkable power over chemical affinity : if the 

 electric spark be passed through a mixture of oxygen and hydrogen gases, 

 it causes them to combine, and water is formed by their union. It is 

 probable that this and some similar effects are produced by the heat 

 which accompanies the electrical spark. 



The action of electricity is much more remarkable in causing decom- 

 posrtion than combination, and especially that form of it which is 

 termed voltaic electricity, or galvanism. The first substance decom- 

 posed by it was water. When two platinum wires are connected with 

 the poles of a voltaic trough, and their unconnected ends are immersed 

 in water, hydrogen gas is evolved at the negative, and oxygen gas at 

 the positive wire. Many other compound bodies have been similarly 

 decomposed; their elements separate at the opposite poles, and the 

 same body always appears at the same pole : thus, in all decompo- 

 sition*, oxygen, chlorine, and the acid* go over to the positive surface, 

 while hydrogen, the metals, and the alkalies are found at the negative 

 .1.1: i. - 



In common electrical attraction, the bodies attract each other in 

 consequence of their opposite electrical states; and in the same 

 manner, in the electro-chemical theory proposed by Sir H. Davy, it is 

 supposed that acids and otbsr substance* which are attracted in 



electrical decompositions to the positive pole, are negatively electrical 

 at the moment of their separation from combination ; and on the 

 contrary, the alkalies, which are found at the negative extremity, are 

 positively electrical. 



It has, however, by no means been proved that chemical affinity ia 

 identical with electrical attraction; and we must yet consider the 

 former as a ] 

 of electrical i 

 pounds, 

 affinities. 



If sulphate of soda be dissolved in a blue vegetable infusion and 

 subjected to voltaic electricity in a glass tube, it is soon found that the 

 fluid at the positive pole becomes red, indicating the presence of an 

 acid, while at the negative it is green, showing the action of an alkali 

 In this case the sulphate of soda is not only decomposed, but it* con- 

 stituents, while under electrical influence, appear to be incapable of 

 recombining ; for, by reversing the position of the tube, or the places 

 of the wires, the fluid which was red will become green, and the green 

 red; thus proving that, while under electrical influence, chemical 

 affinity is suspended, for the acid and the alkali must have passed 

 through the same solution without combining. 



That affinity may be controlled by altering the electrical state of a 

 body, is also proved by Sir H. Davy's very curious experiments on 

 copper-sheathing, which, though they failed from unforeseen causes, 

 are worthy of his genius. It appeared to Sir H. Davy that the copper 

 was oxidised by the atmospheric air in the sea-water, and that then it 

 combined with hydrochloric acid derived from chloride of magnesium, 

 and formed with it subchloride of copper, and hence ensued tin- 

 destruction of the metal. Now, as metals combine with acids only 

 when oxidised, it occurred to Sir H. Davy, that if he could render the 

 copper negative, which is the electrical state of the oxygen, they would 

 not combine. Thi* he effected by bringing the copper into contact 

 with zinc or iron ; these being rendered positive, the copper became 

 negative, scarcely combined at all with oxygen, and was so little acted 

 upon by the chloride of magnesium in sea-water, that when protected 

 by only l-1000th part of iron, oxidisement and conversion into sub- 

 chloride were, to a certain extent, prevented. 



The following will also serve as an example of the reversal of 

 chemical affinity by electricity : Immerse apiece of copper in a solution 

 of nitrate of silver ; the copper is dissolved, and the silver precipi- 

 tated ; if we reverse the experiment, and put a piece of silver into a 

 solution of nitrate of copper, no change is effected ; if, however, the 

 silver while immersed be touched by a piece of platinum, a voltaic 

 circuit is formed, the order of affinity is reversed, the copper is pre- 

 cipitated, and the silver is dissolved. 



Lijht is capable of controlling chemical affinity, both with respect to 

 decomposition and combination. If a mixture of hydrogen and chlo- 

 rine gases be exposed to the sun's rays, they combine with explosion, 

 and form hydrochloric acid ; this effect does not appear to be produced 

 by the heat which accompanies the light, for a considerably higher 

 temperature is not capable of producing the combination. With 

 respect to the decomposing agency of light, it is well known that if 

 pale nitric acid be subjected to it, it suffers decomposition to a certain 

 extent, oxygen gas being evolved ; it is also found that some metallic 

 oxides which retain the oxygen with but slight force of affinity, evolve 

 it and are reduced to the metallic state by the agency of light. Several 

 of the beautiful processes of photography depend upon this effect of 

 light in modifying chemical affinity. 



In concluding the subject of chemical affinity, it is to be observed, 

 that substances often combine in more than one proportion of each, 

 and the quantities are governed by certain laws which have been 

 considered under ATOMIC THEORY. 



CHEMICAL ANALYSIS. Those chemical operations which h.ivu 

 for their object the discovery or determination of the different kinds 

 of matter of which any compound substance consists, are termed 

 analytical operations, and this department of chemical science is called 

 chemical analysis. 



Substances are subjected to chemical analysis for the purpose 

 of determining (1) what constituent* they contain, and (2) the 

 amount of such constituent*. The processes adopted for the first of 

 these objects constitute qualitative those for the second, </uanlit<itn-c 

 analytit. Both qualitative and quantitative analysis may be only 

 partial ; that is, in qualitative analysis it may only be required to 

 detect one or more constituents in a compound substance, or, in quan- 

 titative analysis, the estimation of the amount of one or more con- 

 stituents may be desired. An example of the first is the detection of 

 poisons in animal substances ; of the second, the determination of the 

 commercial value of a manure. It follows that very different methods 

 may have to be employed in the partial examination of a substance 

 cither qualitatively or quantitatively, according to the particular con- 

 ntituunt* which have to be (ought for or estimated. The examination 

 of a substance, on the other hand, may be total, either in respect to 

 the nature or amount of the constituent* it contains ; that is, we may 

 have to determine either qualitatively or quantitatively, the whole of 

 the substances present. 



Further, in both divisions of analysis, different methods have to be 

 employed, according a* it is required to detect or estimate the 

 proximate, or ultimate (elementary) constituents. For instance, in the 



