May I, 1879] 



NATURE 



neutralising one and the same amount of sulphuric acid 

 is concerned. 



The effect of substituting various compound radicles 

 for the hydrogen of ammonia, is well shown in the pheno- 

 mena attending the neutralisation of acid by ammonia, 

 and by those substituted products. The introduction of 

 a C„H2„+i group (CjHj, CH3, &c.) into the ammonia 

 molecule produces a substituted ammonia, the heat of 

 neutralisation of which is the same as that of the parent 

 body ; but if a negative radicle (such as QHj) be substi- 

 tuted for hydrogen, then a compound is produced in the 

 neutralisation of which less heat is evolved than in the 

 neutralisation of the parent body. Thus the neutralisa- 

 tion of hydrochloric acid by ammonia is accompanied with 

 the evolution of 24, 540 units of heat, while the neutralisa- 

 tion of the same acid by aniUne (NHjC^Hg) is accom- 

 panied with the evolution of only 15,000 to 16,000 thermal 

 units. 



When solutions of two salts are mixed under condi- 

 tions such that the products of their mutual action remain 

 in solution, thermal measurements throw veiy consider- 

 able light on the progress of the chemical change. 



The problem presented by the phemonenon now under 

 consideration is one of those which are peculiarly difficult 

 of attack by the older methods. If a third body were in- 

 troduced into the mixture of salts, which should combine 

 with, or render insoluble, one or more of the possible 

 products of the action, a new configuration would be 

 initiated, new chemical changes would probably occur, 

 and we should be unable to say whether the results 

 obtained were really trustworthy representations of the 

 action which had taken place between the members of 

 the original system. 



But measurement of thermal changes involves no dis- 

 turbance of the equilibrium of the reacting chemical 

 system, and at the same time it yields trustworthy infor- 

 mation regarding the changes which have occurred in the 

 distribution of the mass of matter comprising that system. 

 To take an example : — On adding a solution of potassium 

 chloride to dilute hydrochloric acid no thermal change is 

 noticed ; on adding a solution of potassium sulphate to 

 dilute sulphuric acid heat is absorbed, the amount of heat 

 so absorbed increasing with the amount of acid added, 

 until a limiting point is reached. If the solution of 

 potassium sulphate be made more and more dilute less 

 and less heat is absorbed. Now these facts evidently 

 point to the occurrence of two processes of chemical 

 change in the abov ■ reaction, viz., the direct action, 

 formulated HjS04 -|- K2SO4 = 2KHSO4 ; and the inverse 

 action, formulated 



2KHSO4 -f H2O = K2SO4 ->r H2SO4 -f ;i-HjjO 

 We are thus taught to regard this chemical change 

 as dependent on the conditions of the experiment, and 

 further we obtain a glimpse of the decompositions and 

 recompositions which are continuously occurring among 

 the molecules of our seemingly stable compounds. 



If solutions of zinc acetate and sodium sulphate be 

 mixed no thermal change is noticeable, but if solutions of 

 zinc sulphate and sodium acetate be mixed, an evolution 

 of heat occurs, that is to say, a chemical change (or a 

 series of chemical changes) proceeds. Such an experi- 

 ment as this, besides throwing light upon the special 

 chemical change under consideration, leads to a clearer 

 conception of those phrases " strong acid," " weak base," 

 than were generally to be found before the introduction 

 of the thermal method into chemistry. A strong acid is 

 evidently an acid in the formation of the salts of which 

 much heat is evolved, and a weak acid is one in the 

 formation of whose salts little heat is evolved, or heat is 

 absorbed. If therefore the heats of neutralisation of two 

 acids by given bases be known, it may become possible to 

 predict what chemical changes will occur when given 

 salts of those acids are mixed. 

 Attempts have been made from time to time to measure 



the so-called affinities of the elementary atoms. These 

 attempts have been considerably advanced, and the whole 

 problem of affinity has been much defined by applying 

 the results of thermal measurements to chemical reac- 

 tions. 



If chlorine be mixed with hydrogen, and the mixture 

 be exposed to daylight, hydrochloric acid is produced 

 with evolution of a large amount of heat ; the formation of 

 hydrobromic acid from its elements is accompanied with 

 the development of less heat, while heat is absorbed in 

 the formation of hydriodic acid from its elements. These 

 thermal reactions show that more energy changes form 

 in the first than in the second, and more in the second 

 than in the third of these reactions. The amount of 

 energy of motion which is convertible into thermal 

 energy, under fixed conditions, seems, therefore, to 

 measure the mutual affinities of chemical elements. 



But we do not know what is the amount of energy 

 spent in decomposing the molecules of hydrogen and 

 chlorine ; the heat developed in the reaction 



2H + 2CI = 2HCI 

 is therefore the sum of the plus and minus thermal 

 changes during the cycle of chemical changes, the initial 

 and final stages of which are chlorine and hydrogen 

 molecules and hydrochloric acid molecules respectively. 

 Therefore it is evident that thermal measurements do not 

 give data which suffice for determining the absolute 

 affinities of the elements. 



If the elements comprised in a natural group be con- 

 verted into similar compounds — say into oxides — and if 

 that element in the formation of whose oxide the greatest 

 amount of heat is developed be said to have the greatest 

 affinity for oxygen, many remarkable relations may be 

 shown to exist between the affinities and the atomic 

 weights of the elements in such a series. Thus Thomsen 

 has shown that in the group comprising magnesium, 

 calcium, strontium, barium, the affinity for chlorine, 

 bromine and iodine increases with increase of atomic 

 weight, while the affinity of the haloid compounds of 

 these elements for water decreases as the atomic weight 

 of the elements increases. Many more exceedingly 

 interesting results are brought out by Thomsen in the 

 same paper. 



The results of thermo-chemical investigation — a few of 

 which I have endeavoured to sketch in thinnest outline- 

 suggest one or two considerations regarding chemical 

 action in general, and regarding some of those problems 

 which yet remain to be solved by chemical science. 



The older theory of chemical action is based upon the 

 idea that the reacting bodies exert force upon one an- 

 other ; the word affinity has thus a positive meaning. 



Recently the view has gained ground, with some che- 

 mists, that a chemical change is but the outward repre- 

 sentation of a loss of energy occurring within the reacting 

 system ; that no positive force is exerted between the 

 reacting molecules, but that the system, as it were, falls 

 to pieces because the conditions are realised under which 

 a loss of energy is possible. 



The latter view, I think, fails to account for the facts ; 

 there is no doubt that it expresses a truth, but surely only 

 a partial truth. 



General considerations, no less than those derived from 

 thermal measurements, compel us to regard the first 

 action between two elementary molecules as consisting 

 in a decomposition of those molecules with the produc- 

 tion of their constituent atoms, which afterwards combine 

 with the formation cf new molecules. But the decompo- 

 sition of elementary molecules involves the expenditure 

 of energy ; in other words, there is a mutual action and 

 reaction between these molecules. If this stress be re- 

 garded from the point of view of one set of the reacting 

 molecules only, we certainly have positive force exerted. 



It is not a mere negative loss of energy, but a positive 

 action of one kind of molecules upon another kind of 



