IV 



THE BLOOD : FOEMED CONSTITUENTS 



95 



gravity of blood. 



i> carried on can IK- recorded (Fig. 24). After carefully cleaning and drying 



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



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



weighed once more when rilled with the Mood to lie examined. The weighing 



must lie accurate to ^ ingrni. The weight of distilled water at lf> ( '. lieing 



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



same temperature. The areometric meth<i<l, also used 



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



the pycnometric, because it determines the specific 



gravity of the plasma rather than that of the Mood 



in tolo. When only small quantities of blood are 



available, as in clinical researches, the capillary 



pycnome.ter of Schmalz is employed, which consists 



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



in which distilled water and blood 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 

 -pecific 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, 011 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 the>e 

 indirect methods rather determine the specific gravity of the corpuscles than 

 that of the Mood iii M</. 



When care is taken to employ liquids in which the component- of Mood 

 are the, least soluble (for example chloroform and benzol, Hammersch lag'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' Mctlwil Glazed strips of neutral litmus-paper are used, which 

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

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

 rapidly with a tine spray of distilled water. 



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

 acid or alkaline solution of a given strength to lie 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 fiuid. Apart from errors due to 

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



