APPENDIX I, 469 



structural doctrines to the dynamics of Newton, I consider 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 eeem 

 that the whole system of dynamic equilibrium must be destroyed. 



Without entering into the minute analysis of this question, I think it 

 mil 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 substi- 

 tuted for a light one in a chemical compound for example, for magnesium, 

 in the oxide of that metal, an atom of mercury, which is 8 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 necessarily 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 hi the 

 weight of the atoms reproduces some, though not all, of 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 BO, 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 parfe. 



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

 the moon- and let us imagine that the mass of tne 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 epi- 

 cycloidal, 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 un 

 dulatiohs. 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 ampli- 

 tude, those of the moon would also change, and when the mas.s 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. Bu$ a 

 eunilar relation exists between the sun and the earth, because the former is 

 also moving in space. "We way apply these views to the world of atoms, and 



