vinsTM.t.ofDs A\n Tin:/!,- /'i!ori:irrfi:s 27 



CRYSTALLOIDS AND THEIR PROPERTIES 



Crystalloids, or substances tliat tend under favorable conditions to 

 form crystals, aiul whicli diffuse readily tlu'()u<;h most diffusion mem- 

 branes, form a I'elatively small part of tlie total mass of tbe cell, but 

 they are fully as essential as the colloids. The chief representatives 

 of this group that are found usually or constantly in the cell are the 

 inorganic salts, sugar, and the innumerable decomposition products 

 of the proteins, including particularly urea, creatine, purine bases, 

 amino-acids, etc. ^Most of these are by no means so characteristic of 

 living things as are the colloids, sometimes occurring quite inde- 

 pendently of a cellular origin, which the proteins never do. The inor- 

 ganic salts in particular seem quite foreign to living processes, and 

 as they enter and leave the body practically unchanged they are 

 evidently not a source of energy through chemical change. Their 

 importance to the cell lies almost entirely in their physical or physico- 

 chemical properties. The organic crystalloids, although of initritional 

 value, also have physical properties in some respects similar to those 

 of the inorganic crystalloids, and therefore to this extent they exert 

 similar influences, but the essential difference between the organic 

 and the inorganic crystalloids is that all the latter are electrolytes, 

 while many of the organic crystalloids that occur in cells are non- 

 electrolytes. The importance of this distinction lies not in the utility 

 or non-utility of these substances as conductors of electrical cur- 

 rents in the ordinaiy sense, but rather on the existence of those 

 properties which determine their conductive ability. Electrical con- 

 ductivity is an index of ionization, and upon ionization depends the 

 chief influence of the electrohi:es upon vital activities. The impor- 

 tance of this process of dissociation or ionization lies in the fact that 

 with most substances no chemical reaction can occur while the sub- 

 stance is in the non-ionized state. The chemical properties of ioniza- 

 ble substances are produced largely by the ions they liberate on 

 dissociation. As a consecjuence, the physiological effects of electro- 

 lytes are due to their ionic condition, and through the ions that are 

 present in the cell many of its various chemical processes are brought 

 about. Not all substances ionize with the same readiness, which 

 causes a great difference in their properties. The reason that acetic 

 acid is a weaker acid than hydrochloric acid is that it does not 

 ionize to such an extent, and so a corresponding quantity does not 

 introduce as large a number of hydrogen ions into a solution. 

 Larger molecules, as a rule, ionize less than smaller ones of similar 

 nature, e. g., stearic acid ionizes less than acetic acid and therefore 

 is a weaker acid. Likewise the properties of a substance which 

 depend upon its ions will be less marked when it is in a solvent that 

 produces little ionization. For example, bichloride of mercury owes 

 its antiseptic properties to the Hg ions that it sets free when in solu- 



