200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



At the outset let me remind you of an old saying of Plato's, for it 

 sounds the keynote of the lecture: "If arithmetic, mensuration and 

 weighing be taken away from any art, that which remains will not 

 be much." 1 In other words, the soundness of all important con- 

 clusions of mankind depends on the definiteness of the data on which 

 they are based. 



Lord Kelvin said: "Accurate and minute measurement seems to 

 the nonscientific imagination a less lofty and dignified work than 

 looking for something new. But nearly all the grandest discoveries 

 of science have been the rewards of accurate measurement and 

 patient, long-continued labor hi the minute sifting of numerical 

 results." 2 The more subtle and complicated the conclusions to be 

 drawn, the more exactly quantitative must be the knowledge of the 

 facts. 



Measurement is a means, not an end. Through measurement we 

 obtain data full of precise significance, about which to reason; but 

 indiscriminate measurement will lead nowhere. We must choose 

 wisely the quantities to be measured, or else our time may be wasted. 



Among all quantities worthy of exact measurement the properties 

 of the chemical elements are surely some of the most fundamental, 

 because the elements are the vehicles of all the manifold phenomena 

 within the range of our perception. 



Weight is clearly one of the most significant of these properties. 

 The eighty or more individual numbers which we call the atomic 

 weights are perhaps the most striking of the physical records nature 

 has given us concerning the earliest stages of the evolution of the 

 universe. They are mute witnesses of the first beginnings of the 

 cosmos out of the chaos, and their significance is one of the first 

 concerns of the chemical philosopher. 



Mankind is not yet in a position to predict any single atomic 

 weight with exactness. Therefore the exact determination of 

 atomic weights rests upon precise laboratory work; and in order to 

 arrive at the real values of these fundamental constants, chemical 

 methods must be improved and revised so as to free them from 

 systematic or accidental errors. 



What, now, are the most important precautions to be taken in 

 such work ? These are worthy of brief notice, because the value of 

 the results inevitably depends upon them. Obvious although they 

 may be, they are often disregarded. 



In the first place, each portion of substance to be weighed must be 

 free from the suspicion of containing unheeded impurities; otherwise 

 its weight will mean little. This is an end not easily attained, for 

 liquids often attack their containing vessels and absorb gases, crystals 



i Plato, Philebus (trans. Jowett), 1875, vol. 4, p. 104. 



• Sir W. Thomson (Lord Kelvin), Address to British Association, August, 1871, Life, vol. 2, 600. 



