GENERAL PROPERTIES! THE CORPUSCLES. 419 



reaction since it acts simply to change some of the disodium 

 phosphate to monosodium phosphate. 



Na*HP0 4 + HC1 = NaCl + NaH 2 P0 4 



The concentration of bicarbonate of sodium and carbonic acid 

 in the blood is especially important in preserving the weak alka- 

 linity characteristic of the blood. Addition of acid or alkali within 

 limits does not alter the reaction, since the acid reacts with the 

 bicarbonate and disodium phosphate, while the alkali reacts with 

 the carbonic acid or, perhaps, is neutralized by combination with 

 the proteins. This characteristic of the blood as a buffer solution 

 together with the regulating action of the lungs upon the carbonic 

 acid and of the kidney in secreting excess of fixed acids combine, 

 therefore, to maintain a nearly constant reaction of the blood 

 under widely varying conditions of metabolism in health and dis- 

 ease. In the older methods of determining the alkalinity of the 

 blood it was customary to add a dilute acid until a neutral reac- 

 tion was obtained with litmus. In this procedure the acid dis- 

 places the sodium from its combination with carbonic and phos- 

 phoric acid, and the amount of acid necessary for this purpose 

 furnishes no indication of the real reaction of the blood; it deter- 

 mines simply the amount of base combined with these weak acids. 

 This method of titrating the blood is still used to some extent, 

 and the figures obtained give what is known as the " tit rat ion 

 alkalinity" of the blood as distinguished from the true alkalinity, 

 that is to say, the excess of hydroxyl over the hydrogen ions. 



Specific Gravity. The specific gravity of human blood in the 

 adult male may vary from 1.041 to 1.067, the average being about 

 1.055. The most satisfactory method of determining this factor is, 

 of course, to compare the weight of a known volume of blood with 

 that of an equal volume of water, but for observations upon human 

 beings such small quantities of blood must be used that recourse must 

 be had usually to a more indirect method. Perhaps the simplest of 

 the methods suggested is that devised by Hammerschlag.* In this 

 method a mixture is made of chloroform (sp. gr., 1.526) and benzol 

 (sp. gr., 0.889). The mixture is made in such proportions as to 

 have a specific gravity of about 1.055. A drop of blood from the 

 finger is shaken into this mixture; if the drop sinks to the bottom 

 it is evident that the specific gravity of the blood is higher than that 

 of the mixture, and the reverse is true if the drop rises. By adding 

 more of the chloroform or of the benzol, as the case may be, the 

 specific gravity of the mixture may be quickly altered so as to be 

 equal to that of the drop of blood, which will then float in the liquid 

 without a distinct tendency to rise or fall. The specific gravity of 

 the mixture, which is also that of the blood, is then determined by a 

 * Hammerschlag, "Zeitschrift f. klin. Med.," 20, 444, 1892. 



