154 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



by allowing the spectrum to fall on a fluorescent screen, or on a sensitive 

 photographic plate. In order to show absorption bands in this part of the 

 spectrum very dilute solutions of the pigment must be used. 



In order to demonstrate these bands, the telescope of a large spectroscope 

 is removed, and a beam of sunlight or of light from the positive pole of an 

 arc lamp is allowed to fall on the slit of the collimator. The spectrum is 

 focussed on a fluorescent screen.^ The slit is then opened very widely, and 

 the coloured solution is interposed on the path of the beam falling on the 

 slit. 



Oxyhsemoglobin shows a band (Soret's band) between the lines G and H. 

 In haemoglobin, carbonic oxide haemoglobin, and nitric oxide haemoglobin, 

 this band is rather nearer G. Methaemoglobin and haematoporphyrin show 

 similar bands. 



The two preceding figures show the ' photographic spectra ' of haemo- 

 globin, oxyhaemoglobin, and methaemoglobin, and will serve as examples of 

 the results obtained. I am greatly indebted to Prof. Gamgee, to whom we 

 owe most of our knowledge on this subject, for permission to reproduce these 

 two specimens of his numerous photographs. 



9. Preparation of Pure Oxyhaemoglobin.— The following method is described 

 in Stirling's ' Practical Physiology ' (3rd edit. p. 65). Centrifugalise dog's 

 defibrinated blood and pour off the serum. Centrifugalise again with physio- 

 logical saline solution repeatedly until the supernatant fluid contains only 

 traces of proteid. Mix the magma of corpuscles with two or three volumes of 

 water saturated with acid-free ether ; the solution becomes clear. Then add 

 a few drops of 1-per-cent. solution of acid sodium sulphate till the mixture 

 looks tinted like fresh blood, owing to the precipitation of the stromata. 

 These can be separated by centrifugalising. (I have found that they aggre- 

 gate together and can be easily removed by filtration.) Pour off the clear red 

 fluid: cool it to 0° C, add one-fourth of its volume of absolute alcohol pre- 

 viously cooled to 0° C. Shake well, and then let the mixture stand at 5°-15° C. 

 for 24 hours. As a rule the whole passes into a glittering crystalline mass. 

 Filter at 0° C. and wash with ice-cold 25-per-cent. alcohol. Redissolve the 

 crystals in a small quantity of water, and recrystallise as before. The 

 crystals may then be spread on plates of porous porcelain, and dried in a 

 vacuum over sulphuric acid. 



' Fluorescent screens, similar to those in common use in observations made 

 with Bontgen rays, may be made by coating white cardboard with barium platino- 

 cyanide. 



