THE BLOOD 195 



tration are separated by an animal membrane, water will 

 pass from that of lower concentration into that of higher 

 concentration. Hence if blood corpuscles are placed in a 

 series of saline solutions, it will be found that, if the solution 

 is of lower molecular concentration than the corpuscle, fluid 

 will pass in, swell the corpuscle, and perhaps burst it and set 

 free the pigment, while, if the fluid is of higher concentra- 

 tion, the fluid will pass from the corpuscle which will shrivel. 

 Hence the change in the corpuscle is used as a means of 

 determining the osmotic pressure and molecular concentra- 

 tion of various solutions. 



The pigment is Haemoglobin. It constitutes no less than 

 90 per cent, of the solids of the corpuscles. In many animals 

 when dissolved from the corpuscles this substance tends to 

 crystallise very readily. The crystals prepared from the 

 human blood are rhombic plates. When exposed to air they 

 are of a bright red colour, but if placed in the receiver of an 

 air-pump at the ordinary temperature they become of a pur- 

 plish tint. The same thing occurs if the haemoglobin is in 

 solution, or if it is still in the corpuscles. The addition of 

 any reducing agent such as sulphide of ammonia or a ferrous 

 salt also causes a similar change. This is due to the fact that 

 haemoglobin has an affinity for oxygen, which it takes up from 

 the air, forming a definite compound of a bright red colour in 

 which one molecule of haemoglobin links with a molecule of 

 oxygen, Hb0 2 , and is known as oxyhaemoglobin. 



Haemoglobin is closely allied to the proteids, but differs 

 from them in containing 0*42 per cent, of iron. 



When light from the sun is allowed to pass through these 

 solutions certain parts of the spectrum are absorbed, and 

 when the spectrum is examined dark bands the absorption 

 spectra are seen. In a weak solution of oxyhaemoglobin- a 

 dark band is seen in the green and another in the yellow part 

 of the spectrum between Frauenhofer's lines D and E, while 

 the violet end of the spectrum is absorbed (Fig. 101). These 

 bands may be broadened or narrowed by strengthening or 

 weakening the solution. When the oxygen is taken away and 

 the purple reduced haemoglobin is formed, a single broad band 

 between D and E takes the place of the two bands (Fig. 101), 



The property of taking oxygen from the air and of again 



