S. C. BROOKS 175 



substance is constantly exposed to the same light intensity. There- 

 fore if samples of complement are exposed to the light successively 

 for varying periods, and well stirred during the exposure, the apparent 

 order of the reaction will be the true order. 



The transparency of complement was studied in the following way. 

 A 5 per cent solution of fresh guinea pig serum in a physiologically 

 balanced solution^ was made up immediately after the serum was taken 

 from the clot. Except during exposure to light the complement was 

 kept at 0-l°C. The method of titrating the complement has been 

 described in detail in a separate paper.^ It consists essentially in 

 allowing the complement to act on a suspension of specifically sensi- 

 tized sheep erythrocytes suspended in the balanced solution previously 

 mentioned, and noting the amount of each sample of complement 

 required to produce a given amount of hemolysis in a given length 

 of time. The relative efficiency of the various portions of comple- 

 ment was assumed to vary inversely as these amounts and was stated 

 in per cent of the efficiency of an unradiated portion of the same 

 complement. 



It seemed desirable to determine approximately the absorption 

 coefficient of solutions of complement of the concentration used; for 

 from such observations it would be possible to determine what pro- 

 portion of the incident effective light was absorbed in the apparatus 

 used, and also, by analogy with the known absorption of serum, to 

 determine the general region of the spectrum which was causing the 

 photoinactivation. 



To obtain such an approximate value for the absorption coefficient 

 a 5 per cent solution of complement was placed in the inner of two 

 concentric quartz test-tubes, and the balanced salt solution in the 

 annular space between the two tubes; the space was 1.6 mm. wide. 

 This set of tubes was then placed in a vertical position 9.5 cm. from 

 a quartz mercury- vapor arc lamp and rotated on its long axis during 

 an exposure of 3 minutes, which was exactly limited by the inter- 

 position of an opaque screen before and after radiation. Another 

 portion of 5 per cent complement was then radiated in exactly the 

 same manner except that the space between the two tubes (which 



5 Brooks, S. C, /. Med. Research, 1920, xli, 399. 



