IV 



THE BLOOD: FORMED CONSTITUENTS 



95 



Fu;. 24. Pycnometer for de- 

 termining the specific 

 gravity of blood. 



is carried on can be recorded (Fig. 24). After carefully cleaning and drying 

 the pycnometer, it is weighed, first empty, then when filled with distilled 

 water. It is then washed out with alcohol and ether, dried again, and 

 weighed once more when filled with the blood to be examined. The weighing 

 must be accurate to iV mgrm. The weight of distilled water at 15 C. being 

 equal to 1, it is easy to calculate that of blood at the 

 same temperature. The areometric, method, also used 

 in physiology, is more rapid, but is less exact than 

 the pycnometric, because it determines the specific 

 gravity of the plasma rather than that of the blood 

 in toto. When only small quantities of blood are 

 available, as in clinical researches, the capillary 

 pycnometer of Schmalz is employed, which consists 

 of a capillary tube some 12 cm. long by 1^ mm. wide, 

 in which distilled water and 1 tlood are aspirated and 

 weighed in succession. The weighing and calculat- 

 ing are carried out as in the first case. 



Besides these direct methods of ascertaining 

 specific gravity for small quantities of blood, there 

 are other indirect ways which are all based on the 

 principle of obtaining from a more dense and a less 

 dense substance a mixture of the same density as the 

 drop of blood to be examined. This is ascertained 

 when, on introducing the drop of blood into the 

 mixture with a pipette, the drop neither sinks nor 

 rises to the surface. The density of the mixture is 

 then determined with the areometer, and will be that 

 of the blood. The various indirect methods differ among themselves according 

 to the quality of the substances used for the mixture. Fano employs a 

 solution of gum, Roy a solution of glycerin. It should be noted that these 

 indirect methods rather determine the specific gravity of the corpuscles than 

 that of the blood in toto. 



When care is taken to employ liquids in which the components of blood 

 are the least soluble (for example chloroform and benzol, Hammerschlag's 

 method), these methods can be used with approximate accuracy, and they 

 may also be employed for the separate determination of the specific gravity 

 of serum (1028) and of the red corpuscles (1088). 



The following methods are used in determining the chemical reaction of 

 the blood : 



1. Kuhne's Method The drops of blood to be examined are placed in a 

 small dialyser, made of moist parchment, shaped by pressure over a hemi- 

 spherical mould. The drops of blood are introduced into the resulting hollow, 

 and the whole placed in a watch-glass containing distilled water, to dialyse. 

 After a certain time the reaction of this water is tested with litmus-paper. 



2. Liebreich's Method. A drop of the blood to be examined is put on a 

 slab of chalk or plaster, previously saturated with a neutral litmus solution. 

 After a given time the slab is washed with a vigorous spray of distilled water, 

 and the spot where the blood-drop lay is found to be more or less blue in 

 correspondence with its alkalinity. 



3. Zuntz' Method. Glazed strips of neutral litmus-paper are used, which 

 are saturated with a solution of sodium chloride or sodium sulphate. After 

 bringing these into contact with the blood to be examined, they are washed 

 rapidly with a fine spray of distilled water. 



The Titration Methods, which consist in determining the quantity of an 

 acid or alkaline solution of a given strength to be added to the liquid under 

 examination, in order to modify the colour of an indicator, merely give the 

 potential, and not the actual, reaction of the fluid. Apart from errors due to 

 the nature of the indicator, it must be remembered that not only the quantity 



