74 PHYSICAL CHEMISTRY 



pure gelatine jelly or gel, no structure can be seen with the ultra- 

 microscope except at certain concentrations and temperatures, 

 whereas in a suspensoid gel the arrangement of particles into 

 chains and the chains into a network may be distinguished. Ac- 

 cording to Zsigmondy (1912), J^ per cent gelatine shows sub- 

 microns on cooling, first mobile but becoming motionless during 

 gelation. The evidence for a structure in a gel is the sudden in- 

 crease in elasticity during gelation. A gel is not merely a very 

 viscid sol, because agents that increase viscosity induce solation. 

 Fibrin gel seems to be a mass of crystals (W. H. Howell, 1914). 



The emulsoids are probably to be regarded as true solutions, 

 in which the molecules are very large and sometimes aggregated 

 into masses that include more or less water. Some colloids prob- 

 ably have a stronger tendency to aggregate than others. Hemo- 

 globin seems to have little of this tendency when in not too 

 concentrated a solution. Hiifner and Gansser (1907) conclude 

 that it is in true solution. If it be assumed that one molecule 

 of hemoglobin contains one atom of Fe and binds one molecule 

 of CO, its molecular weight is about 16700. Calculated from 

 the osmotic pressure of pure solutions, the molecular weight of 

 horse hemoglobin is 151 15, and of ox hemoglobin 16321. Since 

 one molecule of hemoglobin cannot contain less than one atom of 

 iron, the molecules must be separated in solution in order to 

 exert so high an osmotic pressure. 



Bechhold (1908) has attempted to estimate the size of the 

 particles in emulsoid sols by forcing them through ultrafilters 

 of different grades. These filters are made by impregnating 

 filter paper with an emulsoid sol and hardening it. The size of 

 the pores is determined by the concentration of the sol before 

 being hardened. There are two difficulties with the method: 

 first, the filter may adsorb the colloid, second, some water may 

 be pressed out of the sol in the filter and increase its concentra- 

 tion with enormous increase in viscosity until the flow is stopped. 

 Notwithstanding these difficulties Bechhold has arranged emul- 

 soids in the following series beginning with those with the largest 

 -particles : 



