RICHARDS AND MARK. — THERMAL EXPANSION OF HYDROGEN. 135 



correction was then omitted because it was distinctly beyond the probable 

 accuracy of the work. In the present case, however, the vapor tension 

 of mercurjr at 32.4° (about 0.003 mm., according to Morley)* must be 

 considered, because it is larger than the probable error of the pressure 

 readings. The pressure at 0' is undoubtedly less than 0.001 mm. and 

 therefore negligible even here. It is of course not quite certaiu that 

 the gas is wholly saturated with mercury vapor, but since several hours 

 were allowed to pass before the final readings were taken, the saturation 

 was probably almost complete. The equation giving the desired result 

 is therefore 



_ (p + v _ K - 0.003) (V+y)-PV 

 a P~— pVt 



Here V= 0.272388 liter, V+ v = 0.305423 liter, and K= — 0.582 

 mm., as recounted in the preceding pages. The other quantities in the 

 equation are given below for each experiment. 



The Coefficient of Expansion of Hydrogen. 



Hydrogen was made electrolytically from pure hydrochloric acid, using 

 a platinized platinum cathode and a zinc amalgam anode. It was puri- 

 fied by passage over platinized asbestos, through two tall drying towers 

 filled with solid potash sticks and over phosphoric oxide. The hydrogen 

 entered the bulb from the top, and " dead ends " were avoided as much 

 as possible in the supply system. The method of enclosing within the 

 bulb an amount of gas suitable for the experiment was not modified from 

 that used in the preliminary work. 



In the following table are contained the eight last consecutive and most 

 satisfactory determinations made with pure hydrogen gas. Numbers 1-5 

 iuclusive are measurements on the same hydrogen, and numbers 0-8 

 inclusive are determinations made after a complete repetition of the 

 process of drying and filling the bulb. 



The "probable error" of these results, computed according to ' the 

 method of least squares, is ±0.00000003. This does not mean, however, 

 that the expansion of hydrogen is known to an accuracy of three in the 

 sixth significant figure, because at least one quantity entered into the cal- 

 culation of this coefficient which has a possible error of one in the fifth 

 significant figure ; but it does show that accidental errors of reading tem- 



* Morley, Phil. Mag. (1904), June, 6G3. 



