CHAP. II.] THE BLOOD. 91 



Potassium carbonate added to solutions of haemoglobin precipitates 

 the body without decomposing it, if the temperature be low. 



Solutions of haemoglobin are not precipitated by solutions of lead 

 acetate even after the addition of ammonia, nor by silver nitrate, 

 though these reagents soon lead to its decomposition. 



Alcohol precipitates haemoglobin, the precipitate having at first a 

 red colour, but soon changing to brown, indicating that decomposition 

 has taken place. 



When heated to 70 or 80, dilute solutions of oxy-haemoglobin 

 undergo, for some time, no decomposition ; soon however the liquid 

 becomes turbid and brown, in consequence of the decomposition of the 

 oxy-haemoglobin and the separation of insoluble products. 



These reactions will however be studied with greater advantage after 

 a careful investigation of the optical properties of oxy-haemoglobin, 

 as revealed by an examination of the spectrum of light which has 

 traversed crystals of oxy-haemoglobin, solutions of the body, or which 

 has merely been passed through dilute blood. 



We have used the term oxy-haemoglobin to denote the colouring 

 matter as it exists in the living blood or as it is obtained by the 

 processes we have described : viz. under circumstances in which it 

 exists in combination with a very small proportion of oxygen 

 oxygen which is linked to it by ties so easily broken that it can be 

 transferred to other easily oxidizable bodies existing by its side, that 

 it can be given up when its solutions are gently heated in a Torricellian 

 vacuum, or are agitated at moderate temperatures with large quantities 

 of inactive gases such as nitrogen or hydrogen oxygen which may 

 with appropriateness be spoken of as the respiratory oxygen of 

 haemoglobin. 



Tke ab _ It has long been known that if homogeneous white 



sorption light be passed through certain coloured gases, liquids or 



spectrum of solids, and then through a prism, the spectrum instead 

 oxy-haemo- O f being continuous, is seen to be intersected by dark 

 lines or bands which are termed absorption bands, the 

 spectrum which manifests such bands being designated an absorption 

 spectrum. The situation of such absorption bands, being perfectly 

 constant, often affords a valuable means of identification and a 

 ready means of determining the occurrence and course of changes 

 in composition effected in the body which exhibits them. 



The blood was shewn by Hoppe-Seyler to exhibit when white 

 light is passed through it a very characteristic absorption spectrum, 

 which he was able to shew is identical with the spectrum of pure 

 oxy-haemoglobin, supplying by this discovery the absolute proof that 

 the blood crystals which had by many observers been suspected to 

 be the pure colouring matter of the corpuscles, actually did consist 

 of that substance. 



In examining the absorption spectra of blood or any other solution it is 

 convenient to dilute the liquid sufficiently and then to pour it into a glass 

 vessel with parallel faces, which are a definite width apart. Snch 



