by chloroform, and was first obtained crystalline by Fordos. According to Boland, a reddish-brown colouring matter is found with the pyocyanine, which is insoluble in chloroform but can be extracted by alkalies; this is supposed to be formed from the pyocyanine in the culture of the bacillus. A green fluorescent colouring matter is produced by this microbe when it is cultivated in bouillon.

The oldest known chromogenic micro-organism is Micrococcus prodigiosus or Bacterium prodigiosum, which is the cause of the so-called "bleeding bread," "red snow," etc. The red pigment, termed prodigiosin, is insoluble in water, but is soluble in alcohol, chloroform, and carbon bisulphide. Griffiths gave it the formula Cз8H56NO5, but Scheurlen has obtained analyses which do not agree with this. It has been suggested that this colouring matter is identical with fuchsine, but this statement has been contradicted by Helm and BordoniUffreduzzi.

The red, yellow, and rose-coloured pigments were termed lipochromes by Zopf, the red being termed liporhodines, and the yellow lipoxanthines. They are converted by sulphuric acid into deep blue crystals of lipocyanine. They are closely related to the lipochromes of fat, yolk of egg, etc., and to the carotine of carrots.

naceum.

A violet colouring matter is produced by Bacterium violaceum, and an indigo blue one by Bacterium indigoLankester has named a bacterial colouring matter bacterio-purpurin, which pigment is also found in the sulphur bacteria.

Lastly, green fluorescent pigments are produced by

bacteria, e.g. by Bacilli fluorescens liquefaciens and nonliquefaciens: it has been termed bacterio-fluorescein. Jordan has found that phosphorus and sulphur are essential for its formation, and he has arranged a number of organic compounds in the order of their fluorescegenic value: asparagine, succinic, lactic, citric, tartaric acids, etc.; and he points out that the presence of a methylene group is favourable to fluorescence, but not coincident with dibasicity, nor with the presence of two methylene groups as asserted by Lepierre.

In conclusion, phosphorescence and heat can be produced by bacteria.

CHAPTER XV.

CHANGES IN BLOOD.

AMONG the albumins and allied substances, the transformation which blood undergoes when it is shed, is undoubtedly the change which has been known for the longest time. This change is commonly called coagulation, or "clotting," and it is the result of the action of a ferment.

When blood is drawn and allowed to stand, it quickly becomes gelatinous, but after some hours it is found to consist of a yellowish liquid containing a fibrinous substance, which is red in colour, owing to the mechanical entanglement in it of the red blood corpuscles. If diluted blood be looked at under the microscope, these corpuscles are seen to run together, forming rouleaux, and it was formerly thought that coagulation consisted in the running together of these bodies. In 1772 Hewson showed that these red blood corpuscles took no part in the process, and that the fibrinous substance was formed from the blood plasma, or liquor sanguinis. This observer isolated from the plasma "coagulable lymph," which is now termed "fibrinogen." Buchanan, in 1835, made several observations upon coagulation; he found that hydrocele fluid and pericardial fluid could not

coagulate of themselves, but if he added to them a solution which he obtained from lymphatic glands or from blood clot, coagulation at once set in. He compared this action to that of a ferment, and was therefore the first to suggest that this process was due to a ferment. Denis, in 1859, obtained by another method a substance which he called plasmine, and which on solution could undergo coagulation; this substance was no doubt identical with Hewson's coagulable lymph, or fibrinogen, as it is now called.

Much more complete investigations were carried out by Alexander Schmidt of Dorpat, who rediscovered the facts observed by Buchanan, which had been forgotten. From hydrocele fluid he prepared fibrinogen; and from serum, fibrinoplastin. Of themselves these two substances would not coagulate, but on adding one to the other coagulation resulted. He found that fibrinoplastin contained another substance, whose presence was necessary for the coagulation, but only in 1872 was he able to isolate this by precipitation with alcohol; he called it fibrin ferment, and considered that coagulation was caused by the action of fibrinoplastin on fibrinogen under the influence of fibrin ferment. Fibrinoplastin was subsequently named serum-globulin.

The next experiments were performed by Hammarsten, who showed that fibrin could be formed from fibrinogen and fibrin ferment without the presence of serum-globulin, which had no effect when pure, but contained a factor, when impure, which was necessary for the process; this could be replaced by impure casein or by calcium salts. The fibrinogen is split up during the change,

and does not combine with serum-globulin, as was formerly supposed.

Subsequently, Green found that fibrin ferment contained calcium sulphate, and that calcium sulphate was necessary for the change. Ringer and Sainsbury have shown that any calcium salt can replace the sulphate, and that barium and strontium salts can also be used. Experiments by Horne, in 1896, show that these salts, in the proportion of 0.5 per cent. and upwards, retard the coagulation, barium salts being the most powerful, and calcium salts the least; their influence is overcome by dilution with distilled water, or by the addition of a suitable amount of potassium oxalate; solutions of sodium and potassium chloride of 0.07 per cent. and upwards also retarded coagulation.

Calcium salts are therefore necessary for coagulation; this factor is often made use of when blood is wanted in an uncoagulated condition, by precipitating them as oxalates.

Fibrinogen and calcium salts will not coagulate by themselves, but fibrin ferment must also be present as a third factor, in order that coagulation may take place.

Fibrin ferment is a nucleoproteid; and Pekelharing and Halliburton, who have extracted nucleoproteids from various organs, have shown that these can cause the coagulation of fibrinogen in the presence of calcium salts. The former has therefore suggested that the nucleoproteid transfers calcium with which it combines to fibrinogen, whose calcium salt is insoluble, and constitutes fibrin; the nucleoproteid then combines with more calcium and transfers it again to fibrinogen. This