284 BLOOD AND BLOOD EXAMINATION 



destroyed, and for this purpose the blood is diluted with 0.5 per cent, 

 acetic acid solution. This process has only one disadvantage, viz., that the 

 nuclei of erythroblasts which may be present cannot be differentiated from 

 white blood-corpuscles. In general, however, this error is quite without im- 

 portance. 



The normal number of white blood-corpuscles in a c.mm. of blood varies 

 between 5,000 and 10,000. 



By an estimation of the absolute number of white and red blood-corpuscles 

 we simultaneously learn their proportion to each other, the importance of 

 which we shall describe somewhat more minutely at another place. 



IV. SPECIFIC GRAVITY 



The most exact method, one easily carried out with some practice, is that 

 of Schmaltz. For this purpose we require one or more drops of blood, about 

 0.1 to 0.2 c.c. This is sucked up into small glass capillaries that have been 

 previously weighed upon an accurate chemical scale, noting how much they 

 weigh when empty, and how much after having been filled with distilled 

 water at a temperature of 15° C. Then the weight of the tube filled 

 with blood is determined, and the specific gavity of the blood is cal- 

 culated by dividing its weight by the weight of the same amount of distilled 

 water. 



Hammerschlag's method has been the most extensively used from the fact 

 that it does not necessitate an expensive scale, but only a simple aerometer 

 with divisions from 1.010 to 1.070. 



This process is based upon the physical law : " A body floats in a fluid 

 of the same specific gravity." According to Hammerschlag a medium-sized 

 drop of fresh blood is permitted to fall into a benzol chloroform mixture; if 

 the blood after being dropped into the fluid sinks still lower, it is heavier 

 than the mixture, and by the addition of a corresponding amount of chloro- 

 form an attempt is made to compensate for this. If the drop of blood remains 

 upon the surface it is lighter than its menstruum, which is now made lighter 

 by the addition of more benzol. If, finally, the blood drop remains at the 

 height at which it is first dropped, it possesses the specific gravity of the 

 benzol chloroform mixture, which may easily be read off by the aid of the 

 aerometer. As the blood drop under the influence of the fluid surrounding it 

 readily changes its weight, splitting into many small particles, this process 

 must be carried on very rapidly or fresh drops must be constantly made use of. 

 The benzol chloroform mixture can always be utilized for new estimations, as 

 it is readily freed from blood by filtration. 



In exactness, Schmaltz's pycnometer is decidedly superior to Hammer- 

 schlag's method. 



In normal males the specific gravity of the total blood is 1.059, in normal 

 women 1.056 upon the average. 



In general, the estimation of the specific gravity only complements or con- 

 firms the results obtained by hemoglobin estimation. According to minute 

 exact investigations the amount of hemoglobin and of the specific gravity 



