PRESIDENTIAL ADDRESS. MACKAY Ixvii 



whose diameter is one hundred- thousandth of an inch. In a small 

 box of this size we could pack 16 million molecules close together. 

 The smallest weight which can be weighed on a very good chemical 

 balance is one-hundredth of a milligram. The united weight of 

 one million million million molecules of hydrogen would therefore 

 just be detectable on such a balance." 



It is not surprising that direct confirmation of the existence 

 of bodies liaving such infinitesimal magnitudes has seemed hope- 

 less; and in consideration of this there arose a school of chemists 

 in Germany in the last decade of the last century who, with 

 Ostwald at their head, have attempted to dispense with the atomic 

 theory and rebuild the fabric of chemical theory on a surer founda- 

 tion tlian an hypothesis at once unproved and seemingly incapable 

 of proof. If this attempt were ultimately approved by the 

 world of physicists and chemists, whatever the philosophic gain 

 might be, the practical loss would undoubtedly be great and the 

 progress of physical science retarded. On this account the recent 

 announcement of Professor Eutherford that he has obtained a 

 direct experimental proof of the existence of atoms is one of 

 unusual interest and importance. 



This evidence has come from a quarter from which at one time 

 some timid souls thought present chemical conceptions had much 

 to fear, namely, from the investigation of radio-active matter. 

 The contrary has proved to be the case. Light has already been 

 thrown on atomic structure, of which previously nothing could be 

 asserted, since its problems could not be attacked by ordinary 

 chemical luethods. The new knowledge is thus making conceptions 

 more definite whivh formerly had to be left vague through ignor- 

 ance. It lias not come to destroy but to fulfil. 



The brilliant researches of Eutherford and his co-workers on 

 radium and other radio-active matter have led to the conclusion 

 that the astonishing properties of these substances are only to 

 be explained on the assumption that atoms are to be thought of 

 as complex systems and that the atoms of radio-active substances 

 are unstable, some of them constantly undergoing spontaneous 

 decomposition. In decomposing the atom usually projects particles 

 into the surrounding space while at the same time new forms of 

 matter make their appearance. 



