220 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1912. 



0.50 gram of bromide of radium, the total quantity of emanation can 

 never exceed 0.1 cubic millimeter. 



Let us first see what are the means at our disposal of performing 

 these operations. We may calculate: 



With a good chemical balance 10 -'*=0. 0,001 



With an assay-balance 10-^=0.0,000,01 



With Nernst's micro-balance 10 -g=0.0,000,001 



With the micro-balance constructed by Whytlaw- 



Gray 3 X 10 -9=0.0,000,000,003 



Thespectroscope permits the discovery of helium. 2 X 10-'"=0.0,000,000,000,2 



The sense of smell (for mercaptan) 10-ii=0.0,000,000,000,01 



The electroscope 10-^2^0.0,000,000,000,001 



Everyone is aware how the electroscope has become a much-used 

 means of determining the presence of radium and thorium in rocks. 

 To it M. and Madame Curie owed their discovery of radium, and those 

 who determine the content of rocks in radium confidently distinguish 

 betv/een samples which contain 2.3 X 10"^^ and those containing 

 2.4 X 10'^^ gram per gram of ore. 



Within the last few years we have been devoting our attention to 

 the emanation into which radium changes spontaneously. Owing to 

 the courtesy of the Vienna Academy, I have had at m}^ disposal more 

 than half a gram of bromide of radium, and Mr. Whytlaw-Gray and 

 I have succeeded not only in measuring the quantity of gas which it 

 continually gives off in the gaseous state, but even in determining its 

 volume in the liquid state; in freezing it by cooling it with liquid air; 

 in measuring the wave-lengths of the rays of its spectrum; and in 

 weighing a quantity not exceeding one-tenth of a cubic millimeter. 

 But that is not all. Messrs. Cuthbertson and Porter, my colleagues 

 at University College, have invented an apparatus enabling us to 

 determine the index of refraction of that minimal quantity. There 

 is a proverb in English, "Cut your coat according to your cloth," 

 and its parallel in French seems to be, "Regulate your mouth by 

 your purse." Our purse contained but little of the noble clement, 

 and perhaps this lecture corresponds to that minimal quantity; but 

 I rely upon your indulgence, "making of an egg an ox." 



This bromide of radium, dissolved in water, was contained in a 

 small glass bulb sealed onto a Toepler pump. You know, gentlemen, 

 that under its influence water is decomposed into hydrogen and 

 oxygen. We actually obtained weekly almost exactl}' 25 cubic centi- 

 meters of detonating gas. There is always a small excess of hydrogen, 

 doubtless by reason of the formation of peroxide of hydrogen. This 

 excess is very useful, for it gives after explosion a bubble of hydrogen 

 which carries the emanation and permits of transferring it into vessels 

 for experiment. Moreover, hydrogen is not condensed at the temper- 

 ature of liquid air, while the emanation is deposited on the wails of 



