2 72 THE POPULAR SCIENCE MONTHLY 



Matter, it is held, consists of minute indivisible particles or atoms, 

 of which eighty-one different varieties are at present recognized. 

 These correspond to the chemical elements, oxygen, carbon, sulphur, 

 iron, gold and the rest, by the combination of which all other substances 

 or compounds are found. The atoms of each element are exactly alike, 

 while those of the various elements differ in mass. Thus atoms of 

 sulphur are twice as large as atoms of oxygen, silver atoms are nearly 

 twice as heavy as those of iron. The largest atom known — that of 

 the rare element uranium — is over two hundred times as massive as 

 the smallest, the atom of the elementary gas hydrogen, which is taken 

 as the unit of comparison. 



These atoms are indestructible and can not be converted into one 

 another. However, they are not, like the atoms of Lucretius, 



Solida pollentia sirnplicitate, 



" strong in their solid singleness," but are of complex structure, capable 

 of vibrating in many different ways. From the evidence of the spectro- 

 scope we learn that each kind of atom has its own modes of vibration 

 and is distinguishable from others by these no less than by its mass. 



While the atoms are the fundamental units they can not in most 

 cases exist in isolation, but are drawn together by the forces of chemical 

 affinity into groups which we call molecules. The atom bears to the 

 molecule the relation of the letter to the word on a printed page. 

 While the number of kinds of atom is limited, that of the varieties of 

 molecule is practically unlimited, there being as many kinds of mole- 

 cule as there are substances, or words in the chemical dictionary. The 

 number of atoms in a molecule varies greatly. In a few exceptional 

 cases the atom and molecule are identical. This is the case, for ex- 

 ample, with mercury and with rarer gases of the atmosphere. These 

 elements are the a, I and of chemistry. The inorganic molecules 

 with which we begin our chemical studies are appropriately words of 

 one syllable, containing but a few letters; while some of the organic 

 molecules, with their hundreds or even thousands of atoms, surpass 

 even the creations of Aristophanes and would require the mouth of 

 Gargantua to utter. 



This distinction between atom and molecule is one of the most 

 important characteristics of the theory. The atom it is often said is 

 the unit of the chemist, the molecule of the physicist. To determine 

 the relations of the atoms in the molecule is one of the problems of 

 chemistry; while it is the task of the physicist to form from the inter- 

 actions and motions of the molecules a consistent theory of physical 

 phenomena. To be sure, the boundary between the sciences thus laid 

 down is somewhat arbitrary, and we need not be surprised to find it 

 often overstepped from either side. There is in fact a whole border- 

 land occupied by troops of marauders who style themselves physical 

 chemists or chemical physicists, according to their predilections, and 



