78 CHEMICAL DISCOVERY AND INVENTION 



to the application of the fundamental idea involved, a balance 

 of this construction with some modifications in detail, was used 

 by Professor Sir William Ramsay and Dr. R. Whytlaw Gray in 

 determining the density of the gaseous emanation from radium 

 (q.v.) which Ramsay calls " niton." 



The rationale of the method of weighing is as follows : If a 

 bulb filled with air is at the same temperature and pressure as 

 the air surrounding it the weight of the contained air will be 

 nothing. This is in accordance with the principle of Archimedes. 



FIG. 39. MICRO-BALANCE OF STKKLK AND GKANT. 



If, however, the pressure of the air surrounding the bulb is 

 altered, the sealed up air exerts more or less of its full weight. 

 By suspending a bulb containing a known quantity of air at one 

 arm of a balance, and arranging the whole instrument within a 

 case from which the air can be pumped out to any desired extent, 

 the effective weight of the bulb of air can be changed to any 

 amount desired. 



Temperature changes are eliminated as far as possible, as well 

 as vibration, by mounting the balance on a stone pillar in a 

 cellar, and placing the brass case of the balance inside a large 

 box of bright tin plate. The above diagram will give an idea 

 of the essential parts of the micro-balance of Steele and Grant. 



