August 8, 1889J 



NATURE 



359 



If we bear in mind that for each hydrocarbon serving as a 

 type in the above tables there are a number of corresponding 

 derivatives, and that every compound obtained may, by further 

 methylation, methyienation, acetylenation, and carbonization, 

 produce new hydrocarbons, and these may be followed by a 

 numerous suite of derivatives and an immense number of iso- 

 meric bodies, it is possible to understand the limitless number 

 of carbon compounds, although they all have the one substance, 

 methane, for their origin. The number of substances is so 

 enormous, that it is no longer a question of enlarging the pos- 

 sibilities of discovery, but rather of finding some means of test- 

 ing them, analogous to the well-known two which for a long 

 time have served as gauges for all carbon compounds. 



I refer to the law of even numbers and to that of limits, the 

 first enunciated by Gerhardt forty years ago, with respect to 

 hydrocarbons — namely, that their molecules always contain an 

 even number of atoms of hydrogen. But by the method which 

 I have used of deriving all the hydrocarbons from methane, CH4, 

 this law may be deduced as a direct consequence of the principle 

 of substitutions. Accordingly, in methylation, CH3 takes the 

 place of H, and therefore CHj is added. In methyienation the 

 number of atoms of hydrogen remains unchanged, and at each 

 acetylenation it is reduced by two, and in carbonization by four 

 atoms — that is to say, an even number of atoms of hydrogen is 

 always added or removed. And because the fundamental hydro- 

 carbon, methane, CH^, contains an even number of atoms of 

 hydrogen, therefore all its derivative hydrocarbons will also con- 

 tain even numbers of hydrogen, and this constitutes ih ; law of 

 even numbered parts. 



The principle of substitutions explains with equal simplicity the 

 conception of limiting compositions of hydrocarbons, C„H2„ + 2, 

 which I derived, in 1861,^ in an empirical manner from accumu- 

 lated materials available at that time, and on the basis of the 

 limits to combinations worked out by Dr. Frankland for other 

 elements. 



Of all the various substitutions the highest proportion of 

 hydrogen is yielded by methylation, because in that operation 

 alone does the quantity of hydrogen increase ; therefore, taking 

 methane as a point of departure, if we imagine methylation 

 effected (w-i) times we obtain hydrocarbon compounds con- 

 taining [the highest quantities of hydrogen. It is evident that 

 they will contain 



CH4-f («-i) CH.^, or C„H2„ + o, 



because methylation leads to the addition of CHj to the compound. 

 It will thus be seen that by the principle of substitution — that 

 is to say, by the third law of Newton — we are able to deduce, in 

 the simplest uianner, not only the individual composition, the 

 somerism, and relations of substances, but also the general laws 

 which govern their most complex combinations, without having 

 recourse either to statical constructions, to the definition of 

 atomicities, to the exclusion of free affinities, or to the recogni- 

 tion of those single, double, or treble ties which are so indis- 

 pensable to structurists in the explanation of the composition 

 and construction of hydrocarbon compounds. And yet, by the 

 application of the dynamic principles of Newton, we can attain 

 to that chief and fundamental ol^ject — the comprehension of 

 isomerism in hydrocarbon compounds, and the forecasting of the 

 existence of combinations as yet unknown, by which the edifice 

 raised by structural teaching is strengthened and supported. 

 Besides, and I count this for a circumstance of special importance, 

 the process which I advocate will make no difference in those 

 special cases which have been already so well worked out, such 

 as, for example, the isomerism of the hydrocarbons and alcohols, 

 even to the extent of not interfering with the nomenclature 

 which has been adopted, and the structural system will retain all 

 the glory of having worked up, in a thoroughly scientific manner, 

 the store of information which Gerhardt had accumulated about 

 the middle of the fifties, and the still higher glory of establishing 

 the rational synthesis of organic substances. Nothing will be lost 

 to the structural doctrine, except its statical origin ; and as soon 

 as it will embrace the dynamic principles of Newton, and suffer 

 itself to be guided by them, I believe that we shall attain, for 

 chemistry, that unity of principle which is now wanting. Many 

 an adept will be attracted to that brilliant and fascinating enter- 

 piise, the penetration into the unseen world of the kinetic 



' " Essai d'une thforie sur les limites des combinaisons organiques," par 

 I ). Mendeleeff, 2/1 1 aout 1 861, Bulletin de V Academic i. d. Sc. de St.-P^ters- 

 l>onr^, t. V. 



relations of atoms, to_ the study of which the last twenty five- 

 years have contributed so much labour and such high inventive 

 faculties. 



D'Alembert found in mechanics, that if inertia be taken to- 

 represent force, dyamic equations may be applied to statical 

 questions which are thereby rendered more simple and more 

 easily understood. 



The structural doctrine in chemistry has unconsciously followed 

 the same course, and therefore its terms are easily adopted ; they 

 may retain their present forms provided that a truly dynamical 

 — that is to say, Newtonian — meaning be ascribed to them. 



Before finishing my task and demonstrating the possibility of 

 adapting structural doctrines to the dynamics of Newton, I con- 

 sider it indispensable to touch on one question which naturally 

 arises, and which I have heard discussed more than once. If 

 bromine, the atom of which is eighty times heavier than that of 

 hydrogen, takes the place of hydrogen, it would seem that the 

 whole system of dynamic equilibrium must be destroyed. 



Without entering into the minute analysis of this question, I 

 think it will be sufficient to examine it by the light of two well- 

 known phenomena, one of which will be found in the department 

 of chemistry, and the other in that of celestial mechanics, and 

 both will serve to demonstrate the existence of that unity in 

 the plan of creation, which is a consequence of the Newtonian, 

 doctrines. Experiments demonstrate that when a heavy element 

 is substituted for a light one, in a chemical compound — an atom 

 of magnesium in the oxide of that metal, for example, for 

 mercury, the atom of which is 83 times heavier — the chief 

 chemical characteristics or properties are generally though not 

 always preserved. 



The substitution of silver for hydrogen, than which it is 108 

 times heavier, does not affect all the properties of the substance,, 

 though it does some. Therefore chemical substitutions of this 

 kind, the substitution of light for heavy atoms, need not neces- 

 sarily entail changes in the original equilibrium ; and this point 

 is still further elucidated by the consideration that the periodic 

 law indicates the degree of influence of an increment of weight 

 in the atom as affecting the possible equilibria ; and also what 

 degree of increase in the weight of the atoms reproduces some^ 

 though not all, the properties of the substance. 



This tendency to repetition, these periods, may be likened to 

 those annual or diurnal periods with which we are so familiar on 

 the earth. Days and years follow each other ; but, as they do 

 so, many things change ; and in like manner chemical evolutions, 

 changes in the masses of the elements, permit of much remaining 

 undisturbed, though many properties undergo alteration. The 

 system is maintained according to the laws of conservation in 

 Nature, but the motions are altered in consequence of the change 

 of parts. 



Next, let us take an astronomical case, such for example as the 

 earth and the moon, and let us imagine that the mass of the 

 latter is constantly increasing. The question is, whaj will ther> 

 occur ? The path of the moon in space is a wave-line similar to 

 that which geometricians have named epicycloidal, or the locus- 

 of a point in a circle rolling round another circle. But in con- 

 sequence of the influence of the moon, it is evident that the path 

 of the earth itself cannot be a geometric ellipse, even supposing 

 the sun to be immovably fixed ; it must be an epicycloidal curve, 

 though not very far removed from the true ellipse, that is to say,, 

 it will be impressed with but faint undulations. It is only the 

 common centre of gravity of the earth and the moon which 

 describes a true ellipse round the sun. If the moon were to in- 

 crease, the relative undulations of the earth's path would increase 

 in amplitude, those of the moon would also change, and when 

 the mass of the moon had increased to an equality with that of 

 the earth, the path would consist of epicycloidal curves crossing 

 each other, and having opposite phases. But a similar relation 

 exists between the sun and the earth, because the former is also 

 moving in space. We may apply these views to the world of 

 atoms, and suppose that, in their movements, when heavy ones 

 take the place of those that are lighter, similar changes take 

 place provided that the system or the molecule is preserved 

 throughout the change. 



It seems probable that in the heavenly systems, during incal- 

 culable astronomical periods, changes have taken place and are 

 still going on similar to those which pass rapidly before our eyes 

 during the chemical reaction of molecules, and the progress of 

 molecular mechanics may — we hope will — in course of time, 

 permit us to explain those changes in the stellar world which 

 have more than once been noticed by astronomers, and which are 



