THE RESPIRATORY SYSTEM. 385 



according to the law of pressures. Evidently, then, if the oxy- 

 gen is merely dissolved in the blood, such solution is peculiar, 

 and we shall presently see that this supposition is neither ne- 

 cessary nor reasonable. 



HSlMOGIiOBIN AND ITS DERTVATTVEiS. 



Haemoglobin constitutes about jV of the corpuscles, and, 

 though amorphous in the living blood-cells, may be obtained 

 in crystals, the form of which varies with the animal ; indeed, 

 in many animals this substance crystallizes spontaneously on 

 the death of the red cells. It is unique among albuminous 

 compounds in being the only one found in the animal body 

 that is susceptible of crystallization. Its estimated composi- 

 tion is: 



Carbon 53-85 



Hydrogen 7-33 



Nitrogen 1617 



Oxygen 31-84 



Iron -43 



Sulphur -39 



together with 3 to 4 per cent of water of crystallization. 



The formula assigned is : CoooHBeoOiTsNisiFeSs. The molecu- 

 lar constitution is not known, and the above formula is merely 

 an approximation, which will, however, serve to convey an idea 

 of the great complexity of this compound. The presence of 

 iron seems to be of great importance. If not the essential re- 

 spiratory constituent, certainly the administration of this metal 

 in some form proves very valuable when the blood is deficient 

 in haemoglobin. 



This substance can be recognized most certainly by the spec- 

 troscope. The appearajices vary with the strength of the solu- 

 tion, and, as this test for blood (haemoglobin) is of much prac- 

 tical importance, it will be necessary to dwell a little upon the 

 subject; though, after a student has once recognized clearly the 

 dififerences of the spectrum appearances, he has a sort of knowl- 

 edge that no verbal description can convey. This is easily ac- 

 quired. One only needs a small, flat-sided bottle and a pocket- 

 spectroscope. Filling the bottle half full of water, and getting 

 the spectroscope so focused that the Fraunliofer lines appear 

 distinctly, blood, blood-stained serum, a solution of haemoglo- 

 bin crystals, or the essential substance in any form of dilute 

 26 



