and wired down. The gas exists in the water in a condition of extreme tension, and therefore exhibits a tendency to escape into the atmosphere, in order to relieve the tension; this produces the violent expulsion of the cork when the wire is removed, and if the äerated water be placed in a glass the gas will continue to be evolved until it has almost entirely passed into the atmosphere, and the tension of the gas in the water approximates to that of the atmosphere in which, it should be remembered, the carbon dioxide is, naturally, in very small amount, viz., '04 per cent.

The oxygen of the blood does not obey this law of pressure. For if blood which contains little or no oxygen be exposed to a succession of atmospheres containing more and more of that gas, we find that the absorption is at first very great, but soon becomes relatively very small, not being therefore regularly in proportion to the increased amount (or tension) of the oxygen of the atmospheres, and that conversely, if arterial blood be submitted to regularly diminishing pressures of oxygen, at first very little of the contained oxygen is given off to the atmosphere, then suddenly the gas escapes with great rapidity, and again disobeys the law of

pressures.

Very little oxygen can be obtained from serum freed from blood corpuscles, even by the strongest mercurial air-pump, neither can serum be made to absorb a large quantity of that gas; but the small quantity which is so given up or so absorbed follows the laws of absorption according to pressure.

It must be, therefore, evident that the chief part of the oxygen is contained in the corpuscles, and not in a state of simple solution. The chief solid constituent of the coloured corpuscles is hæmoglobin, which constitutes more than 90 per cent. of their bulk. This body has a very remarkable affinity for oxygen, absorbing it to a very definite extent under favourable circumstances, and giving it up when subjected to the action of reducing agents, or to a sufficiently low oxygen pressure. From these facts it is inferred that the oxygen of the blood is combined with haemoglobin, and not simply dissolved; but inasmuch as it is comparatively easy to cause the hæmoglobin to give up its oxygen, it is believed that the oxygen is but loosely combined with the substance.

Hæmoglobin.--Hæmoglobin is a crystallizable body which constitutes by far the largest portion of the coloured corpuscles. It is intimately distributed throughout their stroma, and must be dissolved out before it will undergo crystallization. Its percentage composition is C. 53.85; H. 7'32; N. 16·17; O. 21·84;

95 S. 63; Fe. 42; and if the molecule be supposed to contain one atom of iron the formula would be C600, H960, N1549 Fe S, 0179 The most interesting of the properties of hæmoglobin are its powers of crystallizing and its attraction for oxygen and other

gases.

Crystals. The hæmoglobin of the blood of various animals possesses the power of crystallizing to very different extents (blood-crystals). In some animals the formation of crystals is almost spontaneous, whereas in others it takes place either with great difficulty or not at all. Among the animals whose blood colouring-matter crys

tallizes most readily are the guinea-pig, rat, squirrel, and dog; and in these cases to obtain crystals it is generally sufficient to dilute a drop of recently-drawn blood with water and expose it for a few minutes to the air. Light seems to favour the formation of the crystals. In many instances other means must be adopted, e.g., the addition of alcohol, ether, or chloroform, rapid freezing, and then thawing, an electric current, a temperature of 140° F. (60° C.), or the addition of sodium sulphate. The hæmoglobin of human blood crystallizes with difficulty, as does also that of the ox, the pig, the sheep, and the rabbit.

Fig. 78.-Crystals of oxy-hæmoglobin-prismatic from human blood.

The forms of hæmoglobin crystals, as will be seen from the appended figures, differ greatly.

Hæmoglobin crystals are soluble in water. Both the crystals themselves and also their solutions have the characteristic colour of arterial blood.

A dilute solution of hæmoglobin gives a characteristic appearance with the spectroscope. Two absorption bands are seen between the solar lines D (which is the sodium band in the yellow) and E (sec plate), one in the yellow, with its middle line some little way to the right of D, is very intense, but narrower than the other, which lies in the green near to the left of E. Each band is

darkest in the middle and fades away at the sides. As the strength of the solution increases the bands become broader and deeper and

Fig. 79.-Oxy-hemoglobin crystals-tetrahedral, from blood of the guinea-pig.

both the red and the blue ends of the spectrum become encroached upon until the bands coalesce to form one very broad band, and only a slight amount of the green remains unabsorbed, and part of the red on still further increase of strength the former disappears.

If the crystals of oxyhæmoglobin be subjected to a mercurial air-pumpthey give off a definite amount of oxygen (1 gramme giving off 159 c. cm. of oxygen), and they become of a purple colour; and a solution of oxy-hæmoglobin may be made to give up oxygen, and to become purple in a similar manner.

This change may be also effected by passing through it hydrogen or nitrogen gas, or by the action of reducing agents, of which Stokes's fluid or ammonium sulphide is the most convenient.

With the spectroscope, a solution of deoxidized or re

Fig. 80.-Hexagonal oxy-hæmoglobin crystals, from duced hæmoglobin is found

blood of squirrel. On these hexagonal plates

prismatic crystals grouped in a stellate manner to give an entirely different not unfrequently occur (after Funke).

dized hæmoglobin.

appearance from that of oxi

Instead of the two bands at D and E we

* Stokes's Fluid consists of a solution of ferrous sulphate, to which ammonia has been added and sufficient tartaric acid to prevent precipitation. Another reducing agent is a solution of stannous chloride, treated in a way similar to the ferrous sulphate, and a third re-agent of like nature is an aqueous solution of ammonium sulphide. NH, H S.

find a single broader but fainter band occupying a position midway between the two, and at the same time less of the blue end of the spectrum is absorbed. Even in strong solutions this latter appearance is found, thereby differing from the strong solution of oxidised hæmoglobin which lets through only the red and orange rays; accordingly to the naked eye the one (reduced hæmoglobin solution) appears purple, the other (oxy-hæmoglobin solution) red. The deoxidised crystals or their solutions quickly absorb oxygen on exposure to the air, becoming scarlet. If solutions of blood be taken instead of solutions of hæmoglobin, results similar to the whole of the foregoing can be obtained.

Venous blood never, except in the last stages of asphyxia, fails to show the oxy-hæmoglobin bands, inasmuch as the greater part of the hæmoglobin even in venous blood exists in the more highly oxidised condition.

Action of Gases on Hæmoglobin.-Carbonic oxide gas, passed through a solution of hæmoglobin, causes it to assume a bluish colour, and its spectrum to be slightly altered; two bands are still visible, but are slightly nearer the blue end than those of oxy-hæmoglobin (see plate). The amount of carbonic oxide taken up is equal to the amount of the oxygen displaced. Although the carbonic oxide gas readily displaces oxygen, the reverse is not the case, and upon this property depends the dangerous effect of coal-gas poisoning. Coal gas contains much carbonic oxide, and when breathed, the gas combines with the hæmoglobin of the blood, and produces a compound which cannot easily be reduced. This compound (carboxy-hæmoglobin) is by no means an oxygen carrier, and death may result from suffocation due to the want of oxygen notwithstanding the free entry of pure air into the lungs. Crystals of carbonic-oxide hæmoglobin closely resemble those of oxy-hæmoglobin. Nitric oxide produces a similar compound to the carbonic-oxide hæmoglobin, which is even less easily reduced.

Nitrous oxide reduces oxy-hæmoglobin, and therefore leaves the reduced hæmoglobin in a condition to actively take up oxygen.

Sulphuretted Hydrogen.—If this gas be passed through a solution of oxyhæmoglobin, the hæmoglobin is reduced and an additional band appears in the red. If the solution be then shaken with air, the two bands of oxyhæmoglobin replace that of reduced hæmoglobin, but the band in the red persists.

Derivatives of Hæmoglobin.

Methæmoglobin.-If an aqueous solution of oxy-hæmoglobin is exposed to the air for some time, its spectrum undergoes a change; the two D and E bands become faint, and a new line in the red at c is developed. The solution, too, becomes brown and acid in reaction, and is precipitable by basic lead acetate. This change is due to the decomposition of oxy-hæmoglobin, and to the production of methemoglobin. On adding ammonium sulphide, reduced hæmoglobin is produced, and on shaking this up with air, oxy-hæmo

H

globin is reproduced. Methæmoglobin is probably a stage in the deoxidation of oxy-hæmoglobin. It appears to contain less oxygen than oxy-hæmoglobin, but more than reduced hæmoglobin. Its oxygen is in more stable combination, however, than is the case with the former compound.

Hæmatin.-By the action of heat, or of acids or alkalies in the presence of oxygen, hæmoglobin can be split up into a substance called Hæmatin, which contains all the iron of the hæmoglobin from which it was derived, and a proteid residue. Of the latter it is impossible to say more than that it probably consists of one or more bodies of the globulin class. If there be no oxygen present, instead of hæmatin a body called hemochromogen is produced, which, however, will speedily undergo oxidation into hæmatin.

Hæmatin is a dark brownish or black non-crystallizable substance of metallic lustre. Its percentage composition is C. 64'30; H. 5·50; N. 9'06; Fe. 882; O. 12:32; which gives the formula Cas, Ho, N., Fe2, 010 (HoppeSeyler). It is insoluble in water, alcohol, and ether; soluble in the caustic alkalies; soluble with difficulty in hot alcohol to which is added sulphuric acid. The iron may be removed from hæmatin by heating it with fuming hydrochloric acid to 320° F. (160° C.), and a new body, hæmatoporphyrin, s produced. Hæmatoporphyrin (Cos, H., Ng, O12, Hoppe-Seyler) may also be obtained by adding blood to strong sulphuric acid, and if necessary filtering the fluid through asbestos. It forms a fine crimson solution, which has a distinct spectrum, viz., a dark band just beyond D, and a second all but midway D and E. It may be precipitated from its acid solution by adding water or by neutralisation, and when redissolved in alkalies presents four bands, a pale band between C and D, a second between D and E, nearer D, another nearer E, and a fourth occupying the chief part of the space between b and F.

Hæmatin in acid solution.—If an excess of acetic acid be added to blood, and

Fig. 81. Hæmatoidin crystals. (Frey.)

the solution is boiled, the colour alters to brown from decomposition of hæmoglobin and the setting free of hæmatin; by shaking this solution with ether, solution of the hæmatin in acid solution is obtained. The spectrum of the ethereal solution (coloured plate) shows no less than four absorption bands, viz., one in the red between C and D, one faint and narrow close to D, and then two broader bands, one between D and E, and another nearly midway between band F. The first band is by far the most distinct, and the acid aqueous solution of hæmatin shows it plainly.

Hæmatin in alkaline solution.-If an alkali be added to blood and the solution is boiled, alkaline hæmatin is produced, and the solution becomes olive green in colour, the absorption band of which is still in the red, but nearer to D, and the blue end of the spectrum is partially absorbed to a considerable extent. If a reducing agent be added, two bands resembling those of oxy. hæmoglobin, but nearer to the blue, appear; this is the spectrum of reduced hæmatin, or hæmochromogen. On shaking the reduced hæmatin with air or oxygen the two bands are replaced by the single band of alkaline hæmatin.

Hæmatoidin. This substance is found in the form of yellowish crystals