134 On the Examination of Mixed Colouring Matters. 



facts connected with the well-known spectrum of blood. If after 

 exposure in a dry state to the air for some weeks, until the haemo- 

 globia has been changed into methaemoglobin, a small quantity of the 

 double tartrate of potash and soda be added to the aqueous solution, 

 and afterwards a very minute portion of the double sulphate of 

 protoxide of iron and ammonia, the methaemoglobin is deoxidized 

 and reconverted into haemoglobin, as described in my late paper " On 

 Blood-stains."* Here then we have a decomposition gradually effected 

 by the atmosphere, and if two different substances had been present, 

 it is extremely probable that they would have varied in the rate of 

 change, so that there would finally have been an alteration in their 

 relative proportion, and thus when deoxidized there would not have 

 been the same relation between the absorption-bands as in fresh 

 blood. I find, however, that the agreement . is complete. More- 

 over, if the colouring matter had been a mixture of two substances, 

 it is extremely probable that there would have been some such 

 variation in their relative amount in the blood of very different 

 animals, as occurs in the colouring matters of different Algae. In 

 order to ascertain whether this is the case, I carefully compared 

 side by side the spectra of human blood and that of the small 

 annehds so common in stagnant pools, and found that the position 

 and relative intensity of the two bands was exactly the same. 



Such then are the principal conclusions that have been forced on 

 my attention in carrying out these investigations. For my own 

 part I must say that they make me think that many of my previous 

 observations require further examination, in order to ascertain 

 whether I have not sometimes believed that I was examining a 

 single substance when it was really a mixture. For the future I 

 shall certainly be quite alive to the importance of the principles 

 described in this paper, and trust that what I have said wUl serve 

 to impress it on others, and assist them in carrying out similar 

 inquiries. 



* ' Monthly Micros. Jouvn.,' vi., 1871, pp. 9-17. 



