CHAP. II.] THE BLOOD. 101 



1. To a solution of a ferrous salt, as for instance of Fe SO 4 + 7H 2 O, a 

 small quantity of tartaric or citric acid is added, and then ammonia until 

 the reaction is alkaline. In consequence of the presence of the vegetable 

 acid, ammonia does not throw down a precipitate of ferrous hydrate, but 

 a clear light-green solution is obtained which readily darkens by absorption 

 of oxygen from the air. Such a solution when freshly prepared exerts a 

 powerfully reducing action upon oxy-haemoglobin. When added in small 

 quantities to a solution of this body or to blood, the colour and spectrum 

 change almost instantly, to be restored again on agitation with air. Often 

 we may observe that on shaking up the reduced solution with air the 

 spectrum of oxy-haemoglobin is restored, though on leaving the solution a 

 moment or two at rest the two bands again disappear, and the single band 

 of reduced haemoglobin appears, proving that when existing side by side 

 with a ferrous salt, reduced haemoglobin can more readily take possession 

 of oxygen than that substance, to which however it afterwards cedes it. 



2. Instead of ferrous sulphate we may employ a solution of a stannous 

 salt prepared after the same fashion, by mixing a solution of stannous 

 chloride, Sn C1 2 , with tartaric acid and then adding ammonia to neutralization. 

 In this case, as in 1, by rendering the liquid alkaline we prevent it pro- 

 foundly decomposing the blood-colouring matter, whilst its oxidizing power is 

 increased. The tin presents the advantage over the iron solution of not 

 becoming deeply coloured as it absorbs oxygen, and therefore not absorbing 

 light passed through it. 



3. The blood or solution of oxy-haemoglobin is boiled at a temperature 

 of 40 C., in a vessel in which a Torricellian vacuum is established by means 

 of a mercurial pump. Very shortly the colour of the liquid and the 

 change in spectrum evidence the complete removal of oxygen. 



4. The blood or solution of haemoglobin is subjected for a long-con- 

 tinued period to the action of a stream, of washed hydrogen or nitrogen. 

 The same apparatus may be employed for this experiment as is used in 

 preparing Haemochromogen. 



Whilst oxy-haemoglobin or its solutions very rapidly undergo 

 change at temperatures above 0C. this is not the case with reduced 

 haemoglobin. Hoppe-Seyler has discovered that when a solution of 

 pure oxy-haemoglobin is sealed up in a glass tube (care being taken to 

 include very little air) after undergoing reduction, as exhibited by its 

 change of colour and spectrum, it suffers no further change and may 

 be kept for years. When such a solution is brought in contact with 

 oxygen oxy-haemoglobin is again formed and may even be crystal- 

 lized. This discovery of Hoppe-Seyler's is of great practical importance 

 to the physiological chemist, as it enables him to prepare standard 

 solutions of oxy-haemoglobin, when temperature and other circum- 

 stances are favourable, and to keep them indefinitely for subsequent use. 



Hoppe-Seyler has also shewn that reduced haemoglobin resists tbe 

 action of pancreatic ferment 1 . 



' l Hoppe-Seyler, "Ueber die Fahigkeit des Hamoglobins der Faulniss sowie der 

 Einwirkung des Pankreasferments zu widerstreben." Zeitschrift f. phys. Chemie, 

 Vol. i. p. 125. 



