538 



SCIENCE 



[N. S. Vol. XXXIV. No. 878 



actual behavior of material and of energy- 

 provides the only safe basis for logical in- 

 ference as to the real essence of things. 

 Faraday was deeply imbued with this con- 

 viction ; and it is widely recognized as the 

 basis of all modern experimental science. 

 The subject of my lecture to-night concerns 

 the methods and general results of several 

 extended series of investigations, planned 

 with the hope of adding a little to the 

 foundations of human knowledge by means 

 of careful experiment. 



At the outset let me remind you of an 

 old saying of Plato 's, for it sounds the key- 

 note of the lecture: "If arithmetic, men- 

 suration and weighing be taken away from 

 any art, that which remains will not be 

 much. ' '- In other words, the soundness of 

 all important conclusions of mankind de- 

 pends on the definiteness of the data on 

 which they are based. 



Lord Kelvin said : "Accurate and minute 

 measurement seems to the non-scientific 

 imagination a less lofty and dignified work 

 than looking for something new. But 

 nearly all the grandest discoveries of sci- 

 ence have been the rewards of accurate 

 measurement and patient, long-continued 

 labor in the minute sifting of numerical 

 results."^ The more subtle and compli- 

 cated 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 rea- 

 son; but indiscriminate measurement will 

 lead nowhere. We must choose wisely the 

 quantities to be measured, or else ®ur time 

 may be wasted. 



Among all quantities worthy of exact 



'Plato, "PhUebus" (trans. Jowett), 1875, Vol. 

 IV., p. 104. 



'Sir W. Thomson (Lord Kelvin), "Address to 

 British Association," August, 1871, Life, II., 600. 



measurement, the properties of the chem- 

 ical 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 sig- 

 nificant 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 sig- 

 nificance is one of the first concerns of the 

 chemical philosopher. 



Mankind is not yet in a position to pre- 

 dict any single atomic weight with exact- 

 ness. Therefore the exact determination 

 of atomic weights rests upon precise labo- 

 ratory 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 pre- 

 cautions 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 sub- 

 stance to be weighed must be free from the 

 suspicion of containing unheeded impuri- 

 ties ; otherwise its weight will mean little. 

 This is an end not easily attained, for 

 liquids often attack their containing vessels 

 and absorb gases, crystals include and oc- 

 clude solvents, precipitates carry down pol- 

 luting impurities, dried substances cling to 

 water, and solids, even at high tempera- 

 tures, often fail to discharge their impris- 

 oned contaminations. 



In the next place, after an analysis has 

 once begun, every trace of each substance 



