238 



Mr. E. G. Young. 



The (NH^SOi was determined by distillation with aeration as in a miero- 

 Kjeldahl determination, using N/20 or N/100 H £ S0 4 in the distillate receiver 

 and titrating excess acid by N/20 or N/100 Na 2 S 2 3 in the iodide-iodate 

 titration. 



The Preparation of Albumin. — Crystalline ovalbumin from one dozen egg 

 whites was prepared by means of (NEL^SOi and N/3 H2SO4, crystallising 

 and recrystallising at the isoelectric point. Samples of the successive 

 recrystallisations were dialysed in the apparatus previously described and the 

 remainder after the third crystallisation treated likewise. 



About 800 c.c. of horse serum were treated as described in a previous 

 communication (6), using the direct method, and the crystals obtained 

 recrystallised three times. This material was used for several of the experi- 

 ments described below. 



Technique of Light Exposure. — Small test tubes of f inch diameter of 

 ordinary clear glass were used. About 5 c.c. of the fluid under examina- 

 tion were placed in a test tube or a small glass spectroscope box and exposed 

 to sunlight or artificial light from which the infra-red rays had been removed 

 by passage through a vessel with parallel sides containing clear water. A 

 5-inch lens with a focal length of 6 inches was used to concentrate the light 

 rays, and it was placed about 5 inches distant from the light source in the 

 case of the arc. The entire solution was exposed to the rays somewhat in 

 advance of the focal point. 



It was highly desirable to obtain a sufficiently powerful source of artificial 

 light with some degree of constancy in place of the capricious sun. The light 

 from a carbon arc, made by the Firma Carl Zeiss for use with their ultra- 

 microscope and mounted on an optical bench, was used for several experi- 

 ments. A current of 10-15 amperes was employed. This will bring about 

 the same effect as sunlight if exposure be for a sufficient length of time. 

 The intensity of the arc is only one-tenth that of mid-day sunlight, and thus an 

 approximately similar result by artificial light requires about five times or more 

 the length of exposure to sunlight. Another disadvantage in the use of the arc 

 is the necessity of changing carbons frequently, for with the current strength 

 used the carbons are completely burned up in 1-| hours, thus necessitating 

 changing three or four times in the course of an experiment with ovalbumin. 



Method of Ph Determinations. — The concentration of hydrogen ions in the 

 various solutions used was determined colorimetrically, using the standard 

 buffer mixtures devised by Clark and Lubs (9). The accuracy of the buffer 

 mixtures was checked electrometrically by means of the potentiometer and 

 the Barendrecht electrode. After some experimentation, the three following 

 indicators were selected as most suitable for the purpose. 



