918 "Research Staff of the Gr. E. C, London, on the 



it was found that the absorption of gas might change the 

 state of the walls of a vessel in a manner not easily reversed, 

 while no evidence could be found that all new vessels, 

 suitably treated, were not in the same state ; the separation 

 of factors (2) and (3) could be made with certainty only if 

 no vessel was used for more than one absorption. In the 

 second place, the easiest way to introduce into a vessel a 

 known amount of phosphorus is to coat a wire with a known 

 amount of red phosphorus, mixed with a trace of a " binder " 

 to secure adhesion, and to heat the wire after it has been 

 introduced into the vessel, (This process of coating wire 

 with predetermined quantities of material is sometimes used 

 in incandescent lamp manufacture and need not be described 

 here.) Since ground joints are inadmissible in absorption 

 experiments, on account of the inevitable presence of grease 

 vapour, the wire had to be sealed in, and the renewal of the 

 wire implies the renewal of the vessel. 



Accordingly it was necessary to use vessels that could be 

 readily obtained by the hundred. Ordinary incandescent 

 vacuum lamps proved entirely suitable for the purpose. In 

 such lamps the discharge which causes the absorption passes 

 between the opposite ends of the filament, maintained at a 

 difference of potential by the current which heats the filament ; 

 the negative end provides the requisite thermionic emission. 

 In almost all the experiments 200-230 volt 40 watt lamps 

 of the usual commercial type were used ; they had tungsten 

 filaments about 1 metre long, about '026 mm. in diameter in 

 bulbs of about 210 c.c. volume. 



It is difficult to coat filaments regularly with weights of 

 phosphorus much less than 0*1 mgm. When smaller 

 quantities of phosphorus were required, they were obtained 

 by filling the lamp with gas, placing it in connexion with a 

 vessel containing white phosphorus at a known temperature, 

 and allowing time for the vapour of this phosphorus to 

 establish its equilibrium pressure in the vessel. The estima- 

 tion of the phosphorus introduced by this method requires a 

 knowledge of the vapour-pressure of phosphorus at tempera- 

 tures below that of the room ; the values assumed have been 

 extrapolated (in a manner which will be described in another 

 communication) from the data of MacRae and Van Voorhis *, 

 which deal with temperatures above 44° 0. It has been 

 found subsequently in many of the experiments that this 

 method is not wholly reliable, because if sufficient time is 



* B. MacKae and C. E. Van Voorhis, Am. Cliem. Soc. J. xliii. p. 547 

 (1921). 



