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.” ! 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 in the minute sifting of numerical 
results.””? 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 nowher@ 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 
1 Plato, Philebus (trans. Jowett), 1875, vol. 4, p. 104. 
2Sir W. Thomson (Lord Kelvin), Address to British Association, August, 1871, Life, vol. 2, 600. 
