DETAILS OF WEIGHING. 



II 



In reducing weights in air to weights in vacuum, the densities at 

 about 2o were assumed to be as given below. The value for silver 

 proceeds from Stas, and was checked by one determination by us of 

 the purest silver (sample W, described later). The value for the 

 brass weights refers to the gold-plated Sartorius weights used in all 

 but a very few of the experiments ; the other set had a slightly higher 

 density, of which account was taken. The correction for 1.000 gram 

 of substance weighed in dry air at 20 and 760 mm. is stated for each 

 substance. 



Especially when sodic chloride or silver chloride was weighed the 

 temperature and barometric pressure were recorded at the time of 

 weighing. In case of significant variations of temperature or pressure 

 the corrections were changed accordingly. The correction is com- 

 puted as follows : 



Correction = w (volume of i gram of substance volume i gram 

 of weights), where w is the weight of i cubic centimeter of air at the 

 given temperature and pressure. The following small table contains 

 a sufficiently accurate statement of this value in convenient form for 

 use with a logarithmic slide rule. 



Value of w, in grains. 



If the air is completely saturated with water vapor the above 

 values should be decreased by 0.000008 at 14 , 0.000010 at 20 , and 

 0.000013 at 24 . But since the air of our balance cases was certainly 

 less than a quarter saturated with moisture, any correction on this 

 account would have been supererogatory. 



*Richards and Stull, in an investigation as yet unpublished. 



