Great importance has always been attached to the investigation of the SPECIFIC GRAVITY of the blood; since the density of the blood affords a measure of the number of corpuscles, and of their hæmoglobin equivalent. It is easy to collect observations, as in the last few years two methods have come into use which require only a small quantity of material, and do not appear to be too complicated for practical clinical purposes. One of these has been worked out by R. Schmaltz, in which small amounts of blood are exactly weighed in capillary glass tubes (the capillary pyknometric method). The other is A. Hammerschlag's, in which, by a variation of a principle which was first invented by Fano, that mixture of chloroform and benzol is ascertained in which the blood to be examined floats, _i.e._ which possesses exactly the specific gravity of the blood[2].
According to the researches of these authors and numerous others who have used their own methods, the specific gravity of the total blood is physiologically 1058-1062, or on the average 1059 (1056 in women). The specific gravity of the serum amounts to 1029-1032--on the average 1030. From which it at once follows that the red corpuscles must be the chief cause of the great weight of the blood. If their number diminishes, or their number remaining constant, they lose in hæmoglobin, or in volume, the specific gravity would be correspondingly lowered. We should therefore expect a low specific gravity in all anæmic conditions. Similarly with an increased number of corpuscles, and a high hæmoglobin equivalent, an increase in the density of the total blood makes its appearance.
Hammerschlag has found in a large number of experiments that the relation between the specific gravity and the amount of hæmoglobin is much closer than between the specific gravity and the number of corpuscles. The former in fact is so constant that it may be represented by a table.
Sp. gravity Quantity of Hæmoglobin (Fleischl's method)
1033-1035 25-30% 1035-1038 30-35% 1038-1040 35-40% 1040-1045 40-45% 1045-1048 45-55% 1048-1050 55-65% 1050-1053 65-70% 1053-1055 70-75% 1055-1057 75-85% 1057-1060 85-95%
In a paper which has quite recently appeared Diabella has investigated these relations very thoroughly, and his results partly correct, and partly confirm those of Hammerschlag. Diabella found from his comparative estimations that differences of 10% hæmoglobin (Fleischl) correspond in general to differences of 4.46 per thousand in the specific gravity (Hammerschlag's method). Nevertheless with the same amount of hæmoglobin, differences up to 13.5 per thousand are to be observed; and these departures are greater the richer the blood in hæmoglobin. Regular differences exist between men and women; the latter have, with the same amount of hæmoglobin, a specific gravity lower by 2 to 2.5.