The plasma of the blood on the contrary--and there is no essential difference between plasma and serum (Hammerschlag)--is much more constant. Even in severe pathological conditions, in which the total blood has become much lighter, the serum preserves its physiological constitution, or undergoes but relatively slight variations in consistence. Considerable diminutions in the specific gravity of the serum are much less frequently observed in primary blood diseases, than in chronic kidney diseases, and disturbances of the circulation. E. Grawitz has lately recorded that in certain anaemias, especially posthaemorrhagic and those following inanition, the specific gravity of the serum undergoes perceptible diminutions[3]. There are still therefore many contradictions in these results, and it is evidently necessary in a scientific investigation always to give the specific gravity of the serum and of the corpuscles, in addition to that of the total blood. A method closely related to the estimation of the specific gravity is the direct estimation of the dried substance of the total blood, "HYGRAEMOMETRY"; the clinical introduction of which we owe to Stintzing and Gumprecht. This method is really supplementary to those so far mentioned, and like them can be carried out with the small amounts of blood obtainable at the bedside without difficulty. Small quantities of blood are received in weighed glass vessels: which are then weighed, dried at 65 deg.-70 deg. C. for 24 hours and then weighed again. The figures so obtained for the dried substance have a certain independent importance; for they do not run quite parallel with those of the specific gravity, amount of haemoglobin or number of corpuscles. The normal values are, for men 21.26%, for women 19.8%. A further procedure for obtaining indirect evidence of the amount of haemoglobin is the DETERMINATION OF THE VOLUME OF BLOOD CORPUSCLES IN 100 PARTS OF TOTAL BLOOD. For this estimation a method is desirable, which allows of the separation of the corpuscles from plasma in blood, that is as far as possible unaltered. The older methods do not fulfil this requirement; since they recommend either defibrination of the blood (quite impossible with the quantities of blood which are generally clinically available); or keeping it fluid by the addition of sodium oxalate or other substances which prevent coagulation. The separation of the two constituents can be effected by simply allowing the blood to settle, or with the centrifugal machine, specially constructed for the blood by Blix-Hedin and Gaertner ("Haematocrit").