BY DR. BURDON-SANDERSON. 193 



freely exposed to air, the haemoglobin is always combined with 

 oxygen (oxyhoemoglobin). Consequently, whenever haemo- 

 globin is spoken of, it is understood to mean oxyhoemoglobin. 

 This oxygen is so loosely combined, that it begins to separate 

 itself from the haemoglobin as soon as the pressure of that 

 gas in the gaseous atmosphere to which it is exposed falls 

 below a certain point, recently determined by Worm Miiller to 

 be about twenty-five millimetres of mercury. So that when 

 blood is subjected to the air-pump, the haemoglobin it contains 

 begins to part with its oxygen as soon as the pressure is reduced 

 to about a sixth of an atmosphere. This is expressed by say- 

 ing that the tension of oxygen in the blood is about twenty- 

 five millimetres Hg. Haemoglobin in solution can be deprived 

 of its oxygen by the addition to the liquid of certain reducing 

 agents (see 18). In animals completely deprived of air, the 

 haemoglobin in the blood loses its oxygen completely in less 

 than a minute (see 111). This is, no doubt, owing to the 

 rapid accumulation in the blood of oxidizable products. When 

 blood or solution of haemoglobin is subjected to the barometer 

 vacuum (see Gases of the Blood), it parts with the whole of 

 its oxygen. Haemoglobin has the property of oxydizing tinc- 

 ture of guaiacum. If a drop of concentrated solution of guaiac 

 resin in absolute alcohol is dropped on to filtering paper, and 

 the alcohol allowed to evaporate, and then a drop of solution 

 placed on the brown spot, a deep blue ring is formed round 

 the edge of the drop. This reaction must not be confused 

 with that observed when fibrin steeped in peroxide of hydro- 

 gen produces a similar effect. In the latter case, all that is 

 shown is, that fibrin decomposes the peroxide; in the former, 

 the reaction affords evidence of the presence of nascent oxy- 

 gen. Action of Carbonic Acid. Blood which has been satu- 

 rated with carbonic oxide is entirely deprived of its oxj'gen, 

 which is replaced b}' an equal volume of carbonic oxide. On 

 this fact is founded the excellent method of Bernard for the 

 gasomctrical determination of the oxygen of the blood (see 

 32). The carbonic oxide combines with haemoglobin in the 

 same way that oxygen does. Action of Oxide of Nitrogen. 

 When oxide of Nitrogen is passed through a solution of blood 

 which has been freed from oxygen, by subjecting it to an atmos- 

 phere of hydrogen in such a manner as to exclude atmospheric 

 air during the process, the dark blood acquires a carmine color. 

 Here, as in the case of carbonic oxide, a new compound is 

 formed with haemoglobin, which crystallizes in the same form 

 as oxy haemoglobin. The solution, however, undergoes no 

 change when treated with reducing agents. Action of Nitrites. 

 Dr. Gamgee has shown that the blood of animals poisoned 

 with nitrites, as e. </., nitrite of amyl, assumes a chocolate 

 color. This color may be observed strikingly if a few drops 

 13 



