94 PHYSIOLOGY CHAP. 



and foresaw many of the theories that are now generally accepted. 

 A few years later (1794) J. Hunter published an extensive work 

 on the blood, which contained not a few new observations and 

 ingenious experiments. Just as the history of the physics and 

 morphology of the blood begins only in the seventeenth century 

 with the discovery of the microscope, so the history of its 

 chemistry only assumes notable proportions at the commencement 

 of the nineteenth century, after Priestley (1775) and Lavoisier 

 (1784) had laid the h'rst methodical principles of modern 

 chemistry. As the precursors of our present science of haema- 

 tology, we may name Berzelius (1808), Prevost and Dumas (1821), 

 Chevreul (1824), Nasse (1842), Simon (1842), Mulder (1849), 

 Lehmann (1850) and many others. 



IV. If we consider the most striking characters of the blood, 

 it is found to be a red fluid (arterial blood, scarlet ; venous blood, 

 dichroic, i.e. dark red in reflected, greenish in refracted, light), 

 somewhat viscous, opaque even in thin layers, faintly salt and 

 sweetish in taste, with a characteristic odour. It is a little 

 heavier than water : the specific gravity of a man's blood varies 

 between 1-057 and 1'066, that of a woman from 1-053 to 1-061. 

 The reaction of the blood circulating in the vessels is alkaline in 

 the normal state ; extracted from the vessels, it becomes neutral 

 and then slightly acid. It must, however, be noted that in all blood 

 reactions, and, generally speaking, in all the fluids of the body, we 

 have to distinguish between the actual or true reaction and the 

 potential reaction. 



Eecent researches in physical chemistry have brought out the 

 fundamental fact that the degree of acidity or alkalinity of a 

 solution is determined by its content of H + ions and OH - ions 

 respectively. Since the actual reaction of a liquid is that which 

 represents its content of free H + or OH - ions, it is necessary 

 in determining it to make use of means which do not alter the 

 numbers of these ions. The potential reaction is that which 

 represents the degree of acidity or alkalinity of a liquid when 

 the electrolytes which it contains are all fully dissociated into 

 their ions. 



The determination of these two kinds of reaction leads in the 

 case of the blood to very different results. While according to 

 the potential reaction the alkalinity of the blood corresponds 

 to a soda solution of 0'2 - 0'4 per cent, according to the actual 



1-3 



or true reaction it would be that of IQQQOOOO-^ ^ s da, which is 

 practically neutral (Farkas). 



The Pycnometric Method is certainly the most exact for determining the 

 specific gravity of blood in animals, and also in man when there is a sufficient 

 quantity of blood to work with. A glass pycnometer is used, which carries a 

 thermometer in its stopper so that the temperature at which the experiment 



