CHAPTER III 

 PHYSICO-CHEMICAL INTRODUCTION 



THE cells of the body are bathed in solutions containing both col- 

 loids and crystalloids. The phenomena exhibited by substances in 

 solutions are closely related to those exhibited by gases, concerning 

 which the basic facts are better known. The fundamental gas laws, 

 proven by experiment, may be grouped, two as physical, and one as 

 chemical, in nature. 



The first law is that the volume occupied by a given mass of gas varies 

 inversely as the pressure to which it is subjected, provided the temperature 

 is kept constant (Boyle's law). Experimental work has shown that for 

 ordinary gases under ordinary conditions Boyle's law may be taken 

 as accurate. 



The second law is that the volume occupied by a given mass of gas, 

 kept under constant pressure, increases as its temperature is raised, and 

 the relative expansion is approximately the same for all gases (Gay- 

 Lussac's law). It is found that the volume of a gas increases by ^ 1 7J 

 of the volume it occupies at C. for a rise of 1 C., always provided 

 the pressure remains the same. 



From these two laws it can be shown that the pressure exerted by a 

 given mass of gas kept at constant volume increases with rising tem- 

 perature in the same proportion as the volume increases at constant 

 pressure. 



The third law is also associated with the name of Gay-Lussac. 

 It is the law of volumes. It states that, when two gases combine with 

 each other to form a third gas. the volumes of the reacting gases are in 

 simple ratios to one another and t) the volume of the gaseous product, all 

 being measured at the same temperature and pressure for example, one 

 volume of hydrogen combines with one volume of chlorine to form two 

 volumes of hydrochloric acid. On one hypothesis (Avogadro's) regard- 

 ing the nature of gases, it is supposed that equal volumes of different 

 gases measured at the same temperature and pressure contain the same 

 number of molecules or ultimate particles. The molecules are not the 

 same thing as the atoms of an element; a molecule may contain one, 

 two, or more atoms, and the element is univalent, divalent, trivalent, 

 according to the number of atoms its molecule contains. 



Diffusion. A characteristic feature of a gas is its ability to occupy 

 with great rapidity any space afforded it. If two vessels containing 

 different gases at the same pressure be put in communication with each 

 other, the gases gradually mingle, each moving from places of high 

 concentration to places of low concentration until the partial prcssuro 



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