THE MOLECULAR COMPOSITION OF MATTER 71 



similar parts which cannot be further divided, as is 

 indicated by the name " atoms." 



The present-day concept of the structure of matter 

 was not firmly established until about 1860. Accord- 

 ing to the ideas accepted since that date matter may 

 be subdivided into small similar parts, called molecules, 

 which are the limit of divisibility for that kind of 

 matter. To subdivide any kind of matter into parts 

 smaller than this perfectly definite particle, is to alter 

 the kind of matter. 



The molecule of starch is one of the largest of which 

 we know, and yet it is so small that the most powerful 

 microscope will not permit us to observe it by eye. 

 We do not know what its shape may be, but if we 

 imagine it inclosed in a sphere we do know that the 

 diameter of this sphere is about one two-hundredth part 

 of the diameter of the smallest particle which we can 

 see with the microscope. The molecule of sugar, which 

 is much larger than many of the other kinds of mole- 

 cules, is only about one ninth the diameter of the starch 

 molecule. The diameter of the molecular spheres for 

 the molecules of the gases which compose the air we 

 breathe is different for each kind, but is about 2X10~ 8 

 cm. 1 How these dimensions may be obtained we shall 

 see hi Chapter XX. 



Matter of most kinds may be divided into particles 

 even smaller than its molecules, but whenever such 

 division takes place the kind of matter is changed. 



1 The symbol 10- 8 means -=^,1.6.0.000,000,01. Very large or very 



small numbers are conveniently expressed "in powers of ten." Since 

 1 cm. is 0.394 in., it follows that the molecular diameter is about 

 8xlO~ 9 inch or eight thousandths of one millionth of an inch. 



