8 



NATURE 



{May I, 1879 



waters. Meanwhile let us be thankful that they have 

 done something to redeem the race to which Cook 

 belonged from the charge of insensibility to his greatness. 



THERMO-CHEMICAL INVESTIGA TION 



THE introduction of a new method of research, or the 

 invention of a new instrument, has repeatedly 

 marked an epoch in the development of more than one 

 branch of natural science. The last few years have wit- 

 nessed the introduction into chemical research of a new 

 method of examining chemical changes, a method which 

 is founded upon the study of those thermal reactions 

 which accompany these changes. 



The older methods of chemical investigation failed to 

 throw any definite light upon many important problems, 

 some at least of which have been brought a step nearer 

 complete solution by the application of the newer method 

 of thermo-chemical measurement. 



When solutions of two salts are mixed, the products of 

 the mutual action of which salts remain in solution under 

 the experimental conditions, it is frequently found impos- 

 sible to determine, by means of the ordinary analytical 

 processes, the chemical distribution of the mass of react- 

 ing matter at the expiry of the experiment. 



Again, there are certain acids which undoubtedly form 

 two series of well-marked salts, but which appear to be 

 capable, under certain ill-defined conditions, of forming a 

 third series of unstable saline derivatives. How to deter- 

 mine the basicity of such acids has long been one of the 

 unsolved problems of chemistry. 



Once more, the ordinary methods of investigation have 

 failed to supply us with any far-reaching generalisation 

 concerning the stabilities of series of compounds. Certain 

 relations have undoubtedly been traced between general 

 chemical properties of compounds, the properties of 

 their constituent elements, and the stability of these com- 

 pounds, but, nevertheless, the shadowing forth of well- 

 marked generalisations, connecting stability of compounds 

 with chemical structure, from which generalisations exact 

 deductions, capable of experimental investigation, might 

 be made, dates from the introduction of the thermo- 

 chemical method of investigation. 



That system of notation which is now employed in 

 chemistry, although of the greatest value, is nevertheless 

 far from being perfect ; it fails to tell anything concerning 

 the changes in forms of energy involved in those changes 

 of distribution of mass which it formulates. Previous to 

 the introduction of the thermo-chemical method little or 

 no exact knowledge regarding these changes of energy 

 was in the possession of chemists. 



Chemists were long aware that certain reactions were 

 possible only under stated conditions of temperature, 

 pressure, &c., but until measurements had been made of 

 the amounts of heat evolved or absorbed in these reactions 

 they were unable to generalise the connection between 

 the conditions of the reactions and the possibility of their 

 occurrence. 



Such are some of the problems which have been at 

 least partially solved by the new method. 



The fundamental position of thermal chemistry may 

 be thus stated : " Every chemical change taking place 

 without the aid of extraneous forces tends to produce that 

 body, or system, in the formation of which the greatest 

 evolution of heat occurs." 



As a deduction from this statement Berthclot formulates 

 his law of maximum work as follows: — "That salt, the 

 formation of which is attended with the greatest evolution 

 of heat, is always produced when those salts, from whose 

 mutual action it may be formed, exist in solution in a con- 

 dition of partial decomposition." 



Many special instances illustrative of these generalisa- 

 tions might be cited ; let one or two suffice. Chlorine 

 decomposes dry sulphuretted hydrogen with formation of 

 hydrochloric acid and separation of sulphur ; iodine does 



not decompose sulphuretted hydrogen under the same 

 conditions. The formation of hydrochloric acid and 

 sulphur in the first change is accompanied with the evo- 

 lution of a considerable quantity of heat ; the formation 

 of hydriodic acid and sulphur, in the second case, would 

 involve the absorption of much heat. If, however, the 

 action of extraneous forces be allowed to supervene, a 

 new condition of equilibrium is attained ; add water to 

 sulphuretted hydrogen and iodine, hydriodic acid and 

 sulphur are produced. But the solution in water of 

 hydriodic acid, which is the potential product of the 

 reaction, involves the evolution of more heat than is 

 absorbed in the reaction itself. 



Iodine scarcely decomposes water, but if sulphurous 

 acid be added to water, iodine is capable of bringing 

 about decomposition, the products of the reaction being 

 hydriodic and sulphuric acids 



(HjO + I2 + H2SO3 = H2SO4 H- 2HI). 



Now it is found that the formation of sulphuric from 

 sulphurous acid is accompanied with the evolution of a 

 considerable amount of heat ; if, then, the decomposition 

 formulated aH^O + 2I2 = 4HI -f O^ be started, the com- 

 bination of the oxygen thus produced with the sulphurous 

 acid present causes the evolution of more heat than would 

 be evolved in any other series of chemical changes which 

 could occur among the bodies present. 



The applications of the thermal method in general 

 chemistry are many and important. I propose briefly to 

 consider some of the results obtained by this method, 

 as shown in the phenomena attending the neutralisation 

 of acids ; in the changes which occur on mixing solutions 

 of two salts which are capable of undergoing decompo- 

 sition with the production of salts themselves soluble 

 under the conditions of experiment ; in the measurements 

 of (so-called) affinities between elementary bodies ; and 

 in one or two other reactions of general interest. 



The neutralisation of an acid by an alkali is attended 

 with the evolution of a constant amount of heat ; in some 

 cases it is noticed that the total amount of heat evolved 

 is independent of the relative quantities of acid and 

 alkali employed, while in other ca-,es the total heat evo- 

 lution may be divided into two equal portions, one half 

 of the whole accompanying the addition of the first por- 

 tion, and one-half accompanying the addition of the 

 second portion of alkah. Those results evidently point 

 to the exhaustion of the available energy of the acid (or 

 alkali) as a phenomenon which takes place in regular 

 stages. The thermal results of neutralisation phenomena 

 are rendered more intelligible when we find that an acid, 

 the neutralisation of which is accompanied with the evo- 

 lution of but one quantity of heat, is also a monobasic 

 acid ; while in the case of a dibasic acid the total amount 

 of heat evolved on neutralisation with alkali is divisible 

 into two distinct portions. Further, a difference is trace- 

 able between the thermal phenomena which attend the 

 neutrahsation of an acid by caustic potash or soda, on the 

 one hand, and by ammonia on the other. 



The reaction formulated 



2KHO + H2SO, = K2SO4 -f 2HsO, 



involves the expenditure of 31,000 thermal units; but the 

 reaction2 NH3 -f H^SOi = (iNHO^SOi is attended with 

 the expenditure of but 28,150 thermal units 



If, however, a compound more strictly comparable with 

 caustic potash in its chemical structure be employed to 

 neutralise sulphuric acid, we find that the heat evolved is 

 equal in both cases ; the reaction 



2N(CH3)40H + H2SO4 = (N(CH3)4)2S04 + 2H2O, 



is attended with the evolution of 31,300 thermal units. 



From the point of view of their thermal reactions, the 

 alkalis (including thallium hydroxide) and the alkaline 

 earths, are strictly equivalent, so far as the power of 



