PROPERTIES OF GELATINE AND GLUE 201 



separate volumes, is known as the "disperse phase," whilst 

 the other is the " continuous phase " or " dispersion 

 medium." The " dispersity " is the degree to which the 

 reduction of the dimensions of the disperse phase has been 

 carried, and is best expressed numerically in terms or 

 " specific surface," i.e. surface area divided by volume, but 

 it is also often expressed as the thickness or diameter of a 

 film or particle. When the dispersity is not high, we have 

 ordinary " suspensions " and " emulsions," which with 

 increasing dispersity merge into the typical colloids. By 

 analogy, colloids have been divided into " suspensoids " and 

 " emulsoids," when the disperse phase is solid and liquid 

 respectively. The classification, however, has not been 

 found satisfactory, for some systems in which the disperse 

 phase is undoubtedly liquid, exhibit characteristic properties 

 of suspensoids, and vice versa. A more satisfactory division, 

 therefore, is found in the presence or absence of affinity 

 between the two phases, the systems being termed " lyophile " 

 and " lyophobe " respectively. If water be the continuous 

 phase the terms " hydrophile " and " hydrophobe " are 

 often used. Broadly speaking, the lyophile colloids corre- 

 spond to the emulsoids, and the lyophobe colloids to the 

 suspensoids. Gelatine is a typical hydrophile colloid. 



Another fundamental idea of colloid chemistry is that the 

 great extension of surface involved in a high dispersity 

 causes the surface energy to be no longer a negligible fraction 

 of the total energy of the system, and that the recent 

 advances in knowledge respecting surface phenomena may 

 be called in to assist in the explanation of the special pro- 

 perties of the colloid state. Particles which exhibit the 

 Brownian movement, about io~ 5 cm. diameter, down to the 

 limit of microscopic visibility (io~ 3 cm.) are termed microns. 

 Particles less than this, but just visible in the ultra-micro- 

 scope (5x10-7 cm.) are termed submicrons. Particles still 

 less, approximately io~ 7 cm., have been shown to exist, and 

 are termed amicrons. The dimensions of molecules such as 

 may exist in true solutions are of the order of io~ 8 cm. A 

 colloid sol may contain particles of various sizes. Thus a 



