CHEMICAL BASIS OF THE ANIMAL BODY. 1293 



are correct they may assist in explaining to some slight extent 

 the difficulties in understanding the causes of the exit of carbon- 

 dioxide from venous blood during its passage through the 

 lungs. (See § 288.) But the possibility here indicated must 

 be received with the greatest caution. 



3. Carbon-monoxide haemoglobin. When a current of car- 

 bon-monoxide is passed through a solution of oxy-haemoglobin 

 the oxygen is driven off and its place taken by the first-named 

 gas. The compound thus formed results, like oxy-haemoglobin, 

 from the union of one molecule of the gas with one of hae- 

 moglobin. It further resembles oxy-haemoglobin in being 

 readily crystallizable in forms isomorphous with those of the 

 former, but the crystals are on the whole less soluble, brighter 

 coloured and more stable than are those of oxy-haemoglobin. 

 The compound of carbon-monoxide with haemoglobin is much 

 more stable than is oxy-haemoglobin, so that the gas is not 

 again expelled by the action of oxygen, a fact which fully 

 explains the fatal result of breathing carbon-monoxide. Finally 

 the spectrum of carbon-monoxide haemoglobin, while very similar 

 at first sight to that of oxy-haemoglobin, differs distinctly from 

 it in the position of its two absorption bands (see Fig. 89, 

 No. 6). The absorption band in the extreme violet between. 

 Gr and II has its centre at w. L. 420. Since the determination 

 of this compound in blood is frequently of considerable impor- 

 tance in medical jurisprudence, many tests for its presence have 

 been devised additionally to the evidence afforded by the spec- 

 troscope. One of the oldest and best consists in adding to the 

 suspected blood twice its volume of caustic soda of sp. gr. 1-3. 

 If carbon-monoxide haemoglobin is present it yields a brill- 

 iant red precipitate, differing entirely in appearance from the 

 brownish-green mass observed if oxy-haemoglobin is present. 



4. Nitric oxide haemoglobin. If a current of nitric oxide be 

 passed through a solution of carbon-monoxide h;emoglobin, the 

 carbon-monoxide is displaced by the former gas. The com- 

 pound thus obtained is still more stable than is carbon-monoxide 

 haemoglobin. It may be crystallized and in solution exhibits 

 two absorption bands very similar to those of oxy-haemoglobin 

 but slightly nearer the red end of the spectrum; these bands 

 are not affected by reducing agents. The position of its ab- 

 sorption band in the extreme violet is the same as that of the 

 carbori monoxide compound. If prepared by passing the gas 

 through ordinary blood, the latter should first be freed from 

 oxygen by a current of hydrogen and care must be taken to 

 neutralize the nitrous acid formed during the process. 



5. Carbon-dioxide haemoglobin. There appears to be no 



