524 Dr. D. Mendeleeff [May 31, 



though it does some. Therefore chemical substitutions of this kind, 

 the substitution of light for heavy atoms, need not necessarily entail 

 changes in the original equilibrium ; and this point is still further 

 elucidated by the consideration that the periodic law indicates the 

 de<^ree 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 main- 

 tained 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, what will then 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 consequence 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 im- 

 movably 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 increase, 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 through- 

 out the change. 



It seems probable that in the heavenly systems, during incalculable 

 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 me- 

 chanics, 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 now so carefully studied. A 

 coming Newton will discover the laws of these changes. Those 

 laws, when applied to chemistry, may exhibit peculiarities, but these 



