86 UNIVERSITY OF MISSOURI BULLETIN 



or a globe of glass as large as a pea, to be magnified up to 

 the size of the earth, each constituent molecule being magni- 

 fied in the same proportion. The magnified structure would 

 be more coarse grained than a heap of small shot, but prob- 

 ably less coarse grained than a heap of cricket balls." 



To give some idea of the number of molecules in a given 

 volume Sir William Crookes (then Mr. Crookes) in 1879 cal- 

 culated that in a bulb 13.5 cm. (about four inches) in diam- 

 eter there were 1,288,252,350,000,000,000,000,000 molecules. 

 The bulb could be exhausted till it contained one-millionth of 

 this number; if the bulb was then punctured the air would 

 enter and the number of molecules required to fill it would 

 be, 1,288,251,061,747,650,000,000,000; if these molecules en- 

 tered at the rate of 100,000,000 per second, the time needed 

 for them to enter would be 408,501,731 years, but if this 

 experiment be made, as it was by Sir William Crookes, the 

 molecules pass in through the puncture at the rate of 3 x 10^^ 

 per second, that is, 300,000,000,000,000,000,000 per second. 



Before the number of molecules in a given volume of 

 gas, which is the same for every gas under the same condi- 

 tions, had been determined, their velocity, for they are in 

 continual motion under ordinary conditions, had been com- 

 puted, which in the air about us and at the temperatures to 

 which we are most accustomed is at speeds of which the arith- 

 metical mean is more than one thousand miles an hour. The 

 molecules of gas encounter other molecules and the mean 

 length of the path between encounters has been determined 

 to be 7.6 X 10"^ meters. This mean free path, although a 

 giant amongst molecular magnitudes, falls short of the small- 

 est interval which the microscope can detect. Two minute 

 specks on the stage of a microscope, even if separated by twice 

 this interval, would nevertheless be blurred together into the 

 appearance of a single object when viewed under the most 

 favorable conditions and through the best of microscopes 

 handled with the utmost skill. 



