420 



NATURE 



{Sept. I, 1887 



communication, I am glad to remember, before the Chemical 

 Section of our Association, meeting that year at Cork, contain- 

 ing an announcement of a discovery which was to revolutionize 

 modern science. This consisted in the determination of the 

 mechanical equivalent of heat, in proving by accurate experi- 

 ment that by the expenditure of energy equal to that developed by 

 the weight of 772 pounds falling through i foot at Manches- 

 ter, the temperature of i pound of water can be raised 1° F. 

 In other word's, every change in the arrangement of the particles 

 is accompanied by a definite evolution or an absorption of heat. 

 In all such cases the molecular energy leaves the potential to 

 assume the kinetic form, or vice versa. Heat is evolved by the 

 clashing of the atoms, and this amount is fixed and definite. 



Thus it is to Joule we owe the foundation of chemical dyna- 

 mics and the basis of thermal chemistry. As the conservation 

 of mass or the principle of the indestructibility of matter forms 

 the basis of chemical statics, so the principle of the conservation 

 of energy ^ constitutes the foundation of chemical dynamics. 

 Change in the form of matter and change in the form of energy 

 are the universal accompaniments of every chemical operation. 

 Here again it is to Joule we owe the proof of the truth of this 

 principle in another direction, viz. that when electrical energy 

 is developed by chemical change a corresponding quantity of 

 chemical energy disappears. Energy, as defined by Maxwell, 

 is the power of d )ing work, and worK is the act of producing a 

 configuration in a svstem in opposition to a force which resists 

 that change. Chemical action produces such a change of con- 

 figuration in the molecules. Hence, as Maxwell says, 'A com- 

 plete knowledge of the mode in which the potential energy of a 

 system varies with the configuration would enable us to predict 

 every possible motion of the system under the action of given exter- 

 nal forces, provided we were able to overcome the purely mathe- 

 matical difficulties of the calculation." The object of thermal 

 chemistry is I0 measure these changes of energy by thermal 

 methods, and to connect these with chemical changes, to esti- 

 mate the attractions of the atoms and molecules to which the 

 name of chemical affinity has been applied, and thus to solve 

 the most fundamental problem of chemical science. How far 

 has modern research approached the solution of this most diffi- 

 cult problem ? How far can we answer the question, What is 

 the amount of the forces at work in these chemical changes? 

 What laws govern these forces ? Well, even in spite of the 

 results with which recent researches, especially the remarkable 

 ones of the Danish philosopher Thomsen have enriched us, we 

 must acknowledge that we are yet scarcely in sight of Maxwell's 

 jiosition of successful prediction. Thermal chemistry, we must 

 acknowledge, is even yet in its infancy ; it is, however, an infant 

 of sturdy growth, likely to do good work in the world, and to 

 be a credit to him who is its acknowledged father, as w ell as to 

 those who have so carefully tended it in its early years. 



But recent investigation in another direction bids fair even to 

 eclipse the results which have been obtained by the examination 

 of thermal phenomena. And this lies in the direction of elej- 

 trical chemistry. Faraday's work relating to conductivity of 

 chemical substances has been already referred to, and this has 

 been since substantiattd and extended to pure substances by 

 Kohlrausch. It has been shown, for example, that the resist- 

 ance of absolutely pure water is almost an infinite quantity. 

 But a small quantiiy <.f an acid, such as acetic or butyric acid, 

 greatly increases the conductivity ; but more than this, it is 

 possible by determination of the conductivity of a mixture of 

 water with these tw o acids to arrive at a conclusion as to the 

 partition of the molecules of the water between the acids. Such 

 a partition, however, implies a change of position, and there- 

 fore we are furnished with a means of recognizing the motion of 

 the molecules in a liquid, and of determining its amount. Thus 

 it has been found that the hindrance to molecular motion is more 

 affected by the chemical character of the liquid lhan by physical 

 characters such as viscosity. We have seen that chemical change 

 is always accompanied by molecular motion, and further evi 

 dence of the truth of this is gained from the extraordinary 

 chemical inactivity of pure unmixed substances. Thus pure 

 anhydrous hydrochloric acid does not act upon lime, whereas 

 the addition of even a trace of moisture sets up a most active 

 chemical change, and hundreds of other examples of a similar 

 kind might be stated. Bearing in mind that these pure anhy- 



' " The total energy of any miter' al rystem is a quaatity which can 

 neither be increased nor diminished by any action between the parts of the 

 system, though it may be transformed into any of the forms of which 

 energy is susceptible." — Maxwell. 



drous compounds do not conduct, we are led to the conclusion 

 that an intimate relation exists between chemical activity and 

 conductivity. And we need not stop here ; for a method is 

 indicated indeed by which it will be possible to arrive at a 

 measure of chemical affinity from determination of conductivity. 

 It has indeed been already shown that the rale of change in the 

 saponification of acetic ether is directly proportional to the 

 conductivity of the liquid employed. 



Such wide-reaching inquiries into new and fertile fields, in 

 which we seem to come into nearer touch with the molecular 

 state of matter, and within a measurable distance of accurate 

 mathematical expression, leads to confident hope that Lord 

 Rayleigh's pregnant woids at Montreal may ere long be realized : 

 " It is from the further study of electrolysis that we may expect 

 to gain improved views as to the nature of chemical reactions, 

 and of the forces concerned in bringing them about ; and I 

 cannot help thinking that the next great advance, of which we 

 already have some foreshadowing, will come on this side." 



There is, perhaps, no branch of our science in which the 

 doctrine of the Daltonian atom plays amoe conspicuous part 

 than in organic chemistry or the chemistry of the carbon com- 

 pounds, as there is certainly none in which such wonderful pro- 

 gress has been made during the last fifty years. One of the most 

 striking and perplexing discoveries made rather more than half 

 a century ago was that chemical compounds could exist which, 

 whilst possessing an identical chemical composition, that is con- 

 taining the same percentage quantity of their constituents, are 

 essentially distinct chemical substances exhibiting different 

 properties. Dalton was the first to point out the existence of 

 such substances, and to suggest that the difference was to be 

 ascribed to a different or to a multiple arrangement of the con- 

 stituent atoms. Faraday soon afterwards proved that this sup- 

 position was correct, and the research of Liebig and Wohler on 

 the identity of composition of the salts of fulminic and cyanic 

 acid gave further confirmation to the conclusion, leading Fara- 

 day to remark that " now we are taught to look for bodies com- 

 posed of the same elements in the same proportion, but differing 

 in their qualities, they may probably multiply upon us." How 

 true this prophecy has become we may gather from the fact that 

 we now know of thousands of cases of this kind, and that we are 

 able not only to explain the reason of their difference by virtue 

 of the varying position of the atoms within the molecule, but 

 even to predict the number of distinct variations in which any 

 given chemical compound can possibly exist. How large this 

 number may become may be understood from the fact that, for 

 example, one chemical compound, a hydrocarbon containing 

 thirteen atoms of carbon combined with twenty- eight atoms of 

 hydrogen, can be shown to be capable of existing in no less than 

 802 distinct forms. 



Experiment in every case in which it has been applied has 

 proved the truth of such a prediction, so that the ch .-mist has no 

 need to apply the cogent argument sometimes said to be used by 

 experimentalists enamoured of pet theories, " When facts do not 

 agree with theory, so much the worse forthe facts " ! This piwer 

 of successful prediction constitutes a high-watermark in science, 

 for it indicates that the theory upon which such a power is based 

 is a true one. 



But if the Daltonian atom forms the foundation of this theory, 

 it is upon a knowledge of the mode of arrangement of these 

 atoms and on a recognition of their distinctive properties that 

 the superstructure of modern organic chemistry rests. Certainly 

 it does appear almost to verge on the miracul jus that chemists 

 should now be able to ascertain with certainty the relative posi- 

 tion of atoms in a molecule so minute that millions upon 

 millions, like the angels in the schoolmen's discussion, can 

 stand on a needle's point. And yet this pr-oc.ss of orientation is 

 one which is accomplished every day in out laboratories, and 

 one which more than any other has led to results of a startling 

 character. Still, this sword to open the oyster of science would 

 have been wanting to us if we had not taken a step farther than 

 Dalton did, in the recognition of the distinctive nature of the 

 elemental atoms. We now assume on good grounds that the 

 atom of each element possesses distinct capabilities of combina- 

 tion : some a single capability, others a double, others a triple, 

 and others again a fourfold combining capacity. The germs of 

 this theory of valency, one of the most fruitful of modern chemical 

 ideas, were enunciated by Frankland in 1S52, but the definite 

 explanation of the linking of atoms, of the tetrad nature of the 

 carbon atoms, their power of combination, and of the difference 

 in structure between the fatty and aromatic series of compounds, 



