j2 FUNDAMENTAL PRINCIPLES 



Those ions which are metanic in nature carry positive charges, and those 

 which are nonmetalUc carry negative charges; they are termed, therefore, 

 positive or negative ions. Table salt (NaCl) in solution separates mo 

 sodium (Na) and chlorine (CI) ions; sodmm sulphate (Na^SO 4), into 

 Na and SO4 ions, SO4 being a radical. Na is a positive ion; CI and bU4 

 are negative. When, by evaporation of the solution or by precipitation, 

 the substance which is in solution is made to reappear again m sohd form, 

 the ions combine, and the charges neutralize one another and disappear. 

 This separation of ions in solution is known as eledrolyszs, or dzssonahon; 

 different substances show great differences in the degree to which this 

 occurs Sugar and other substances which are nonconductors do not 

 show much dissociation. Acids by dissociation produce H ions; bases, 



19"colloids.-Many thin membranes occur in the bodies of animals 

 in which openings exist of very minute size; similar membranes can be 

 artificially produced. Whenever two different hqmds are m contact 

 with the two sides of such a membrane, the Uquids tend to mmgle by the 

 passage of molecules or atoms through the openings. When one or both 

 of these two Uquids contain soUds, other liquids, or gases m solution, the 

 particles of these substances also may pass through the same openings 

 By the use of these membranes we can distinguish two categories of 

 substances. Those substances which when they are m solution are 

 capable of passing through such a membrane are said to be crystaloidal 

 and If thy normally exhibit this character are often o.lled cry siaUo^ds 

 while those which will not pass through are termed colloidal and called 

 colloids. But many crystalloids may be made to assume ^ ^^ ^^daj 

 condition. Of course it is a matter of relative size of particles and 

 openings but in general it is true that crystalloids are substances which 

 eS in atoms o^ as molecules of very small size in a solution, while 

 colloids are substances which exist as particles and molecules of larger size 

 are dispersed in hquids in the form of suspensions, and do not form true 

 so utions. These colloidal suspensions are thicker or more like glue than 

 are crystalloidal solutions. From crystalloidal solutions the subs ance 

 are easily obtained in crystalline form, but this is not true of col oidal 

 Tspensions. Oils and fats and proteins, such as the a bumen w^^^^^^^^^^ 

 the white of eggs, are colloidal. This separation of ^^''^ff'^Z^^ 

 loidal substances by membranes is known as dialysts and the membranes 

 which effect the separation, as dialyzing membranes or ^^^^^^j;- 



Other membranes are found in the body which under similai conditions 

 permit some hquids or gases to pass through them and P--nt other 

 Lm so doing, but the passage takes place as a result of solution m the 

 colidal memk-ane and not through openings in it. Such a -embrane is 

 termed a semiper^eaUe membrane. This process is known as 0^08^2^ 

 the force behind the movement is called osmotic pressure. In the human 



