16 HEMOGLOBIN 



however, by which the precipitate may be retained in the form of a 

 suspension, so fine that to the naked eye the appearance of a solution 

 is simulated. 



If some colloid material is present — gum arable serves excellently 

 for the purpose — in a solution of turacin in -4 per cent, ammonia, 

 the addition of acid does not now produce an obvious precipitate, 

 the particles are "protected" — a phenomenon well known in colloid 

 chemistry — by the colloid and give the appearance of being in solu- 

 tion whilst the type changes from B to A. 



The points which I have tried to set forth are illustrated in the 

 above figure. From above downwards are three conditions under 

 which type A is found and then two under which type B appears. 

 On looking at the figures the reader must remember that the charts 

 are very far from representing photographs of the spectra, the lines 

 shown are only a few Angstrom units in width and represent as 

 nearly as may be the position of maximum density of the bands. 

 The bands themselves are broad, but it is not possible to say how broaxi, 

 because that depends on the concentration of the solution, the 

 intensity of the illumination, etc. With these points in mind let us 

 consider what will occur if a solution of turacin is made slowly to 

 precipitate. 



To start with, before the precipitation commences only type B 

 will be seen. As the precipitation proceeds a spectrum of type A 

 will appear and will be superposed on type B. Now because the 

 a-bands are relatively narrow and because the a-band of type A 

 is widely separated from that of type B, the composite spectrum will 

 show two a-bands which I shall call a^ and a^ . But in the composite 

 spectrum j8^ and jS^ overlap, so that one band is seen in the /S position, 

 with its position of maximum density intermediate between that of 

 P^ and ^B- We then have for a time a three-banded spectrum in 

 which, as the precipitation proceeds, the A elements increase in dis- 

 tinctness at the expense of the B elements until, in the end, the A 

 elements only are visible. 



So much for turacin. To what extent is it possible to show a 

 similar phenomenon in the case of other metalloporphyrins ? The only 

 one of these which has been studied is the iron compoimd. If the 

 reader will look on page 9 he will find the graphical formula for 

 hsemin. In that formula the chlorine may be replaced by OH yielding 

 a base C34H3204N4Fe . OH which m the absence of oxygen may be 

 reduced to C34H3204N4Fe . The material so formed, like turacin. 



