Jan. 3, 1884] 



NA TURE 



In the ordinary chemical notation almost every chemi- 

 cal change is represented as much simpler than it really 

 is ; no indication is given of the fact that in most cases 

 an excess of one or other of the reacting substances must 

 be used. Thus the reaction usually written 



AgCl + HI (gas) = Agl + HCl 

 would more correctly represent the distribution of the 

 reacting bodies were it written 

 *-AgCl + yHI=;i-AgI+yHCl + (.r-l)AgCl + (jr - l)HI. 



If it is assumed that in the thermal study of a chemical 

 reaction allowance is made for all the purely physical 

 changes which accompany the chemical change, for the 

 influence of the masses of the reacting substances and 

 for the possible formation and decomposition of molecular 

 groups during the reaction, there yet remains the con- 

 sideration that heat is lost or gained to the system in the 

 decompositions and formations of elementary molecules, 

 which decompositions and recompositions may form parts 

 of the entire change under examination. Thus, take the 

 comparatively simple reaction 



2H2O + Cl4 = 4HCl + O2; 

 expanded thermally we have 



r = 4[H, Ci] + [O, O] - 2[H-, O] + 2[C1, CI]. 

 Even the apparently most simple case, the union of two 

 elements, is more comple.x than at first sight appears. 

 [H-, C1-] = 44,000 -j- simply tells that 2 grams of hydrogen 

 combine with 71 grams of chlorine, and that 44,000 

 gram-units of heat are evolved. But if we wish to apply 

 these data to questions concerning the affinity of chlorine 

 for hydrogen, we must remember that affinity is the name 

 given to the stress between atoms when regarded from 

 the point of view of one kind of the reacting atoms. 

 Hence, remembering that the molecules of hydrogen and 

 chlorine are diatomic, we must amplify the equation 

 [H-, Cl^] = 44,000 -f, and write it thus— 



r = 2 [H, CI] - [H, H] - [CI, CI] == 44,000 + ; 

 but the value to be assigned to two of the terms in this 

 equation are unknown. Until we are able to assign ap- 

 proximate values to the thermal changes accompanying 

 the decompositions of elementary molecules and the 

 combinations of elementary atoms, we shall not be in a 

 position to apply thermal data to the subject of affinity, 

 provided, that is, we use this term in its most precise 

 meaning. 



The statement of Berthelot in the " Essai de M^- 

 canique Chimique," that the quantity of heat evolved in 

 a reaction measures the sum of the physical and chemical 

 changes which occur in that reaction, and furnishes a 

 measure of the chemical affinities, is evidently untrue if 

 we assign any ])recise meaning to the term "affinity." 

 But if we use this term in a wide sense as summing up 

 the various actions and reactions (other than those which 

 are purely physical) which together constitute any given 

 chemical change, then we may perhaps say that thermal 

 measurements of comparable reactions are also relative 

 measurements of the affinities of the reacting substances. 

 It is in some such sense as this that the term "affinity " 

 is used by Thomsen in his thermal researches on the 

 relative affinities of the non-metallic elements (vol. ii.). 



It is worthy of remark that Thomsen's arrangement of 

 the commoner acids in order of relative affinities agrees 



very well with that given by Ostwald as the result of 

 his investigations conducted on altogether different lines 

 and by very different methods. 



If thermal measurements of chemical changes really 

 represent the sums of various partial changes, some of which 

 have a positive and others a negative value, then it 

 becomes doubtful whether any practical result is to be 

 looked for from the application of Berthelot's imv of 

 luaximum -nior/c, which runs thus : — 



"Every chemical change, accomplished without the 

 addition of energy from without the system, tends to the 

 formation of that body or system of bodies the produc- 

 tion of which is accompanied by evolution of the maximum 

 quantity of heat." 



Thomsen puts this "law'' in a somewhat different 

 form : he says, " Every simple or complex reaction of a 

 purely chemical kind is accompanied by evolution of 

 heat." Thomsen explains that by a purely chemical 

 process he means one which is accomplished without 

 addition of energy from sources external to the system, 

 and consists in the " striving of atoms towards more stable 

 equilibrium.'' But there are, I think, two principal ob- 

 jections to this statement. Actions " of a purely chemical 

 kind," as thus defined, do not actually occur except as 

 parts of cycles of reactions wherein are included changes 

 not of a ''purely chemical kind." And, secondly, we 

 have at present no means of measuring the thermal values 

 of those purely chemical actions — i.e. on Thomsen's view, 

 atomic actions — but are obliged to include their values in 

 the total value assigned to the complete cycle of operations 

 which we term a chemical reaction. 



Thomsen has it is true attempted to assign thermal 

 values to the decomposition of the molecule of carbon 

 into atoms and the recombination of atoms of carbon to 

 form molecules. The pages of Nature are scarcely 

 suitable for a detailed discussion of Thomsen's methods ; 

 it seems to me, and I think to some others who have 

 tried to follow Thomsen's arguments, both in the second 

 volume of his " Untersuchungen " and also in the original 

 papers in the Berichtc and elsewhere, that these argu- 

 ments really bristle with assumptions, and that a com- 

 parison of the results deduced by Thomsen with the 

 actual calorimetric measurements obtained by himself 

 and others is sufficient to throw grave doubt on the 

 validity of those assumptions on which his arguments are 

 based. One general result which appears to me to follow 

 from Thomsen's investigation is that the time has come 

 when we may with great advantage give up such expres- 

 sions as " the carbon atom has four bonds," " such or 

 such atoms are held by double links," and indeed the 

 whole of that unscientific pseudo-dynamical nomenclature 

 whi h has grown up around the vague and indefinable 

 conception of atomic bonds. 



There are many other points of interest in Thomsen's- 

 "Untersuchungen"; but I have said enough I trust to 

 show the importance and the remarkable suggestiveness 

 of these volumes ; and also to establish the statement 

 that the great advances of the future in chemistry are to 

 be looked for, not so much in the domain of organic 

 chemistry as in the application of the methods and 

 generalisations of the science of matter and motion to 

 the problems which we call chemical. 



M. M. Pattison MuiR 



