206 PHYSICAL SCIENCE 



Rutherford and Geiger, as explained on p. 192, 

 have detected by the throw of the needle of an 

 electrometer the flight of each a particle shot 

 through a gas in a strong electric field. C. T. R. 

 Wilson has shown how to make the tracks of 

 individual a particles visible by the lines of cloud 

 formed upon them. And, as we know, a particles 

 are positively electrified helium atoms. 



Turning to chemistry, we are again impelled 

 to molecular conceptions by the familiar evidence 

 on which rests Dalton's atomic theory. It is 

 true that at one time an alternative explanation, 

 based on the principles of energetics alone, was 

 put forward. As we have seen in Chapter IV., 

 mixtures possessing a maximum or minimum 

 melting or boiling-point change their state with- 

 out change in composition of either phase. The 

 particular composition at which this mode of 

 change occurs depends in general on the physical 

 conditions, such as pressure. If, however, as a 

 limiting case, variation in conditions is without 

 effect, the system would be classed as a compound 

 or an element — a compound ii the constancy 

 extends only over a limited range, an element if 

 no known variation of conditions will alter the 

 composition. This attempt to connect matter and 

 energy was premature and has been discarded. 

 But modern views of atomic structure and other 

 physical problems have suggested another, and 

 more deep-seated relation between the two funda-^ 

 mental concepts of matter and energy. To this 

 point we shall return. 



Truth may possess many aspects, but, since 

 the time of Dalton, it has been safe to accept 



