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.
Should the parallelism between the number of red blood corpuscles and the amount of hæmoglobin be considerably disturbed, the influence of the stroma of the red discs on the specific gravity of the blood will then be recognisable. Diabella calculates, that with the same amount of hæmoglobin in two blood testings, the stroma may effect differences of 3-5 per thousand in the specific gravity.
Hence the estimation of the specific gravity is often sufficient for the determination of the relative amount of hæmoglobin of a blood. It is only in cases of nephritis and in circulatory disturbances, and in leukæmia, that the relations between specific gravity and quantity of hæmoglobin are too much masked by other influences.
The physiological variations which the specific gravity undergoes under the influence of the taking in and excretion of fluid do not exceed 0.003 (Schmaltz). From what has been said, it follows that all variations must correspond with similarly occurring variations in the factors that underlie the amount of hæmoglobin and the number of corpuscles.
More recent authors, in particular Hammerschlag, v. Jaksch, v. Limbeck, Biernacki, Dunin, E. Grawitz, A. Loewy, have avoided an omission of many earlier investigators; for besides the estimation of the specific gravity of the total blood, they have carried out that of one at least of its constituents, either of the corpuscles or of the serum. The red blood corpuscles have consistently shewn themselves as almost exclusively concerned with variations in the specific gravity of the total blood; partly by variations in number, or changes in their distribution; partly by their chemical instability; loss of water and absorption of water, and variations in the amount of iron.
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 anæmias, especially posthæmorrhagic and those following inanition, the specific gravity of the serum undergoes perceptible diminutions[3].