April 3, 1902J 



NA TUBE 



523 



not be given here, but it is probable that many problems of 

 organic life may find solution by the application of such 

 principles. If we generalise the conditions which have been 

 described, we see that the most essential condition is the 

 presence of a metastable system which only passes to the more 

 stable state by its inherent forces, after the way has been 

 opened to it. The nucleus of the other pha.se is not the cause 

 of the reaction in the .sense that Robert Mayer uses this word, 

 because it does not supply the free energy necessary for the pro- 

 cess, but is only the means of starting a process which goes to 

 completion by its own forces after it has once been started. We 

 may now be quite clear that the same thing must hold for 

 all other cases of contact action. It is just the want of 

 proportionality between the quantity of the catalytically acting 

 substance and the amount of change which makes it a necessary 

 condition that the changes produced catalytically supply the 

 necessary energy themselves. The recognition of this is appar- 

 ent with Berzelius when he says that by such a process "a 

 greater electrochemical neutralisation " is effected. One of 

 the most prominent investigators of catalytic enzyme actions 

 states that eiidothermic actions can never take place in this 

 way. This is quite incorrect, for since endothermic reactions 

 take place of themselves, that is, without catalytic influence, 

 there is no reason for supposing that they should not take 

 place under enzyme influence. It is, indeed, true that under this 

 influence no reactions are possible in which there is a reduction 

 of free (but not of total) energy. In other words, no reactions 

 are possible under the influence of catalysts that could not 

 take place in their absence without a breach of one of the laws 

 of energy. The same is to be said about the view that cata- 

 lysts can only have a decomposing and not a synthetic action. 

 Organic chemistry affords many examples to the contrary. It is 

 only necessary to refer to syntheses by the catalytic influence of 

 potassium ethylate. 



(2) Catalysis in homogeneous mixtures. 



We now come to the largest and theoretically most important 

 class of contact actions. The explanation given for the first 

 class cannot here be applied, for while the essential thing in 

 the first class was the appearance of a new phase, this is now 

 excluded by definition. We get the right standpoint for re- 

 garding the new problem by adhering to the condition which 

 has just been laid down for all systems that undergo contact 

 action : — the system must not represent a stable condition, for 

 such a system can undergo no change without the addition of 

 energy. How, then, do unstable systems behave when they are 

 homogeneous? The answer is that homogeneous unstable 

 systems cannot exist otherwise than in a state of change. The 

 supersaturated solution can, if the supersaturation is within 

 certain limits, be kept unaltered for an indefinitely long time, 

 when properly protected. A liquid, however, which without 

 the addition of free energy can produce other soluble liquid 

 products cannot be kept without forming these products. This 

 change may proceed, no doubt, very slowly, so slowly that with- 

 out tedious investigation specially directed to the point no 

 change at all can be observed, but the surest basis for general con- 

 clusions that we know — the laws of energetics — demand, as a 

 matter of fact, that the change must take place. They prescribe 

 no numerical value to the velocity ; they only demand that the 

 velocity be not strictly zero, but that it shall have a finite value. 



By these considerations we gain now for this present case the 

 definition of a catalyst. 



A catalyst is any substance which alters the velocity of a 

 chemical reaction without appearing in the final product. 



In this definition we purposely avoid expressing any view as 

 to what is the cause of such influence. We must, indeed, take 

 care only to state that for all catalytic actions, causes of the 

 same kind are at work. We seek at present only a definition 

 which will be helpful towards a scientific investigation of the 

 question. 



That the definition just given fulfils this purpose will be at 

 once recognised, for it leads directly to inquiry into the numerical 

 value of the acceleration or retardation, and the dependence of 

 this on the nature and concentration of the catalysts, the 

 temperature, the presence of other substances, &c. It is 

 evident, and it must be insisted upon, that all attempts to 

 propound theories for the cause of catalytic phenomena will 

 remain useless until quantitative measurements of the kind 

 referred to have been made. 



Regarding catalysis in the sense just defined, it is an 

 extremely common phenomenon, one, as a matter of fact, 



NO. 1692, VOL. 65] 



that appears wherever the velocity of chemical reaction can be 

 measured. The well-known researches of Menschutkin furnish 

 a capital example. He proved that for a number of different 

 kinds of reactions, the velocity varied between very consider- 

 able limits, according to the solvent. We must regard this 

 efi'ect of the solvent as catalytic. We do not thereby prejudice 

 the further question as to the actual cause ; whether, for 

 example, there is combination between the solvent and the 

 reagents, leading to an alteration of the active masses and so to 

 alteration of the velocity. 



We may bridge over the gap between influences of this kind 

 and others in which vanishingly small quantities of an addi- 

 tional substance alter the velocity to a very high degree. 

 Hitherto only the exaggerated cases have been called catalytic, 

 but as the difference is only quantitative a division between 

 them is not justifiable. So far the cases which have been in- 

 vestigated and measured are those in which great influences are 

 exerted by small quantities of material, but restricting ourselves 

 even to these, the number of cases is extraordinarily great. We 

 are especially indebted to the labours of Schbnbein for an almost 

 endless list of such reactions. Schcinbein certainly did not know 

 that he was dealing only with accelerations of slow spontaneous 

 processes ; on the contrary, he looked upon them as being 

 initiated by catalysts. We are thus confronted with the task 

 of subjecting the raw material of this untiring and original 

 investigator to a quantitative revision, a labour which would 

 engage the united activities of a w"hole series of workers. 



I will not attempt to enumerate such cases : I will only remark 

 that there seems to be no kind of chemical reaction which cannot 

 be catalytically influenced, and no chemical substances, whether 

 elements or compounds, which cannot act catalytically. Like- 

 wise the answer to the question already asked by Berzelius, as 

 to whether there are general or specific catalysts, must be 

 that both kinds exist. Whilst, for example, the presence of 

 hydrogen ions accelerates most chemical reactions, so that they 

 must be regarded as catalysts of great generality, there are, 

 especially among the enzymes, specific catalysts which only 

 exert their accelerating action on perfectly definite substances. 

 The other question of Berzelius, as to whether different 

 catalysts can produce different products from the same substance 

 or substances and whether different possible reactions in a given 

 system can be effected in different ways by different catalysts, 

 must, I think, be answered in the affirmative, although no special 

 experiments have been made in this direction. 



The first theory of catalytic phenomena was set up by Liebig 

 and for the purpose of showing that Berzelius' conception was 

 superfluous. Liebig regarded catalysis as the direct consequence 

 of the law of inertia. His statement is as follows : — " The 

 cause lies in the capacity which a substance undergoing decom- 

 position or combination, that is, in chemical activity, possesses 

 of arousing in a body in contact with it the same chemical 

 activity, or of making the body susceptible of the same kind of 

 change. This capacity is best illustrated by a burning substance, 

 by means of which similar activity is aroused in other bodies 

 when we bring the burning one in contact with them." 



Liebig has obviously not been felicitous in this explanation. 

 His example recoils upon him, for in order to ignite the sub- 

 stance we do not require a burning body, but a hot one, and it 

 is a matter of indifference whether it is hot by means of chemical 

 reaction or from any other cause, for example an electric 

 current. Such objections as these were raised to Liebig's view, 

 and he found himself obliged to give his hypothesis a different 

 form. He expressed his view in reference to sugar fermentation 

 in the following words : — "Just as heat is capable of disturbing 

 the statical moment in the elements of many chemical com- 

 pounds, so this'Can happen by means of a substance of which the 

 elements are themselves in a condition of disturbed equilibrium ; 

 the motion which its atoms possess is imparted to the atoms of 

 the elements of sugar ; they cease to persist in that state in 

 which they form sugar and arrange themselves according to 

 their special attractions." 



This hypothesis of molecular vibrations has enjoyed great 

 popularity, and represents even to-day the view of many people, 

 especially of tho-se who have not taken part in the investigations. 

 It has the special advantage that it cannot be disproved, since 

 it is altogether inaccessible to a test. The scientific unfruitful- 

 ness that lies in such a " theory " was the less perceived, inas- 

 much as the development of the rest of chemistry was taking 

 place in a direction where the employment of molecular hypo- 

 the.ses had the value of a very important scientific auxiliary. 



