868 Transactions of the American Institute. 



Chemists of the atomic school happily avoid the vexed question 

 concerning the indivisibility of matter, by denning an atom as the 

 smallest quantity of an element which can enter into the composition 

 of a ponderable molecule ; and the molecule, whether made up of one r 

 two or more elements, as the smallest quantity which can exist in 

 a free state. However, a certain individuality must be assigned to 

 the single atom, for a chemical decomposition requiring its transfer 

 from one molecule to another involves its isolation in transitu. The 

 absolute weight of the sixty-three different atoms cannot be ascer- 

 tained ; nevertheless, their relative weights have been determined 

 with great care. 



It is difficult to. arrive at any clear notion concerning the size of an 

 object so minute as to be forever invisible under the most powerful 

 magnifier. As an example of the conclusions regarding molecules, 

 founded on microscopic scrutiny, that of the celebrated Ehrenberg 

 may be cited. " x " Without attempting to make a close approximation 

 toward its actual dimensions, his researches led him to infer that the 

 diameter of an atom (the molecule of the chemist) was considerably 

 less than six millionths of a line. Quite recently Sir "W. Thomson, 

 in a paper on " The Size of Atoms,"f presented four lines of argu- 

 ment founded on experiments of physicists, which all lead to substan- 

 tially the same estimate of the dimensions of molecular structure. 

 He says : 



"Jointly they establish, with what we cannot but regard as a very 

 high degree of probability, the conclusion that, in any ordinary liquid,, 

 transparent, solid, or seeming opaque solid, the mean distance between 

 the centers of contiguous molecules is less than the hundred-millionth 

 and greater than the two thousand-millionth of a centimetre. To 

 form some conception of the degree of coarse-grainedness indicated 

 by this conclusion, imagine a rain-drop, or a globe of glass as large as 

 a pea, to be magnified up to the size of the earth, each constituent 

 molecule being magnified in the same proportion. The magnified 

 structure would be coarser-grained than a heap of small shot, but 

 probably less coarse-grained than a heap of cricket-balls." 



From these deductions of Thomson some idea may be formed of 

 minute molecular grouping ; and I venture the suggestion that, in 

 regard to size, the smallest bullet would probably stand about mid- 

 way between the glomeramen minimu?n and " the great globe itself. 1 ' 



Beyond this point of extreme tenuity, where matter first exhibits 

 that property which is revealed in visible forms, we are forced to con- 



*Pogg. Annalen., xxiv., 35. f Nature, No. 22, vol. i, p. 551. 



