APPENDICES 649 



molecules of the water. As in the case of two miscible fluids in contact a 

 uniform mixture is produced. The sugar is said to go into solution in the 

 water. The sugar is termed the solute and the water the solvent. The 

 solvent is usually a liquid. The solute may be a solid, e.g. sugar in water, 

 a liquid, e.g. alcohol in water, or a gas, e.g. CO, in water. 



In their erratic flight some of the molecules of the sugar in solution 

 impinge upon the solid sugar. When the free molecules of the solute have 

 reached such a degree of concentration in the solvent that as many 

 molecules are striking the solid as are leaving it, no higher concentration is 

 possible, and the result is termed a saturated solution. If the solution 

 be heated the kinetic energy of the molecules of the sugar both in 

 solution and in the solid mass is increased and more break free from the 

 solid, and consequently the degree of concentration is increased. This 

 explains why, under the influence of heat, a substance dissolves more 

 rapidly and a higher degree of concentration is obtained. 



According to the foregoing description a solution can be regarded as a 

 homogeneous mixture of two substances, the molecules of which are in free 

 movement throughout the whole of the mixture. 



Colloids (see p. 12). 



The essential character of the colloidal state consists, in the existence 

 together in a physical combination, of two substances, one of which is in 

 the form of ultra-microscopic particles dispersed in the other. The 

 dispersed particles are separated from each other by the containing sub- 

 stance which forms a continuous film or medium surrounding the particles. 

 Each particle has thus a surface of contact with the substance forming the 

 continuous medium. At that surface the dispersed particle is internal 

 and the continuous substance external. In this colloid state, therefore, 

 matter is in two forms or "phases" — (1) dispersed or internal, and (2) 

 continuous or external. 



When the two substances are immiscible fluids the colloid complex 

 forms an emulsion. In protoplasm, however, the condition is not simply 

 a dispersion of one fluid in another immiscible fluid, since in it matter may 

 exist in all degrees between the liquid and the solid condition. Protoplasm 

 is termed an Emulsoid. 



The main points of difference between a solution and a colloid may 

 be noted : — 



Solution. Colloid. 



Molecules small. Molecules very large or molecules 



aggregated into particles. 



Free movement of molecules of Dispersed particles separated by con- 



solute which possess kinetic tinuous phase. Little or no 



energy. movement, therefore little or no 



kinetic energy of particles. 



No surface phenomena. Large size of molecules introduces 



phenomena of surface tension and 

 adsoiption between the dispersed 

 and continuous phases. 



