I04 PLANT PHYSIOLOGY 



dentally, forms a good quantitative test of its presence. It is made by 

 mixing pyrogallic acid in solution with a surplus of caustic potash, when the 

 pyrogallate is formed thus: C 8 H 3 (OH)3 + 3KOH=C 6 H 3 (OK)3 + 3H 2 O. 

 This solution is an eager absorbent of oxygen (forming a soluble dark -brown 

 substance, C w H w O, fl ), and, of course, owing to the presence of surplus caus- 

 tic potash, of carbon dioxide. The most diverse proportions of the constitu- 

 ents have been recommended by different workers, from the .25 g. (solid) 

 pyrogallic, and .6 g. caustic potash in 10 cc. water (viz., i to 2.4 to 40), found 

 by WEYL to be the optimum (Ann. der Chemie, 205, 255, and Ber. d. d. Chem. 

 Ges., 14, 2659 and 2667), to the 5 g. pyrogallic acid, 120 [sic] g. caustic potash 

 in 95 cc. water (viz., i to 24 to 19), of HEMPEL (" Methods of Gas Analysis," 

 115). The WEYL solution evolves carbon monoxide (about 1/30 of the 

 oxygen absorbed) if the gas contains over 28% of oxygen, but the HEMPEL 

 solution quite prevents this (CLOWES in Proc. Chem. Soc., 1895, 200), -which 

 is a reason for the use of this strength. I have myself somewhat thoroughly 

 tested the different solutions from the present point of view, and find that 

 one consisting of i part pyrogallic acid to 5 parts caustic potash to 30 parts 

 of water gives an optimum between absorptive power and convenience in 

 preparation; it will absorb about 10 times its own bulk of oxygen (one 

 molecule pyrogallic acid with three of caustic potash absorbs three atoms of 

 oxygen, that is, i gram pyrogallic acid absorbs nearly one-third of a gram of 

 oxygen; BERTHELOT, Comptes Rendus, 126, 1898, 1459), though in practice 

 it is best to allow only half that quantity or less (one-fourth recommended 

 by HEMPEL). The absorption should take place at a temperature above 

 15 (below which it is slow), and, if the solution is constantly agitated or 

 flowing, only about three to five minutes is required, though much longer 

 is needed if the solution is still. Since the solution spoils almost immediately 

 in air, and in a few hours in light, it should be made up just before using, 

 which may conveniently be done by mixing at the moment of use equal parts 

 of a i in 15 solution of freshly made pyrogallic acid with a 5 in 15 solution 

 of caustic potash, made up long enough before use to have lost its heat and 

 air. Nor should the experiment be left standing for hours, since the solu- 

 tion may ultimately give off carbon dioxide. The solution may best be 

 applied to the gas by use of the reagent tubes described on the next page. 

 (2) PJwsphorus. For some purposes phosphorus is a more convenient 

 absorbent of oxygen than is pyrogallate of potash, but it cannot be used 

 where living tissues are in any way concerned, is slower in its action, and is 

 more dangerous to use, both because of the bad burns it can inflict, and 

 also because of its tendency to ignite in the air. It is best employed in the 

 slender sticks supplied for this especial purpose, which are used in a wire 

 cage held on a wire support; this is inserted into a chamber, or reagent 

 tube over water, and then is surrounded for some time, at a temperature 

 above 15, by the air to be analyzed. During this time, as well as at all 

 other times, the phosphorus should be in darkness to prevent formation of 

 an amorphous coating over it. In absorbing oxygen white fumes are given 

 off, through formation of anhydride of phosphorous acid, P 4 O 6 , and this 

 immediately dissolves in the water after the equation P 4 



