16 REPORTS ON THE STATE OF SCIENCE.—1918. 
Since the precipitation of one colloid by another may take place 
within a relatively narrow range of concentrations, it is not 
surprising that people have postulated the existence of definite 
chemical compounds in order to account for the precipitates. 
The literature on the subject is in a worse state even than that on 
the so-called basic salts. For instance, ferric arsenate, aluminum 
silicate, stannic phosphate, and cupric eosinate are not formed 
under ordinary conditions by precipitation from solutions. The 
precipitation is due to the mutual coagulation of two colloids, 
though the resulting precipitate may simulate a compound to the 
extent that it may be peptised without change under certain 
conditions. 
While the theory of peptisation and precipitation, as outlined, 
accounts satisfactorily for most of the facts, there are certain 
points which are not covered by it at present. A mixture of the 
two water-peptisable colloids, gelatine and gum arabic, is said to 
behave exactly like casein.°° Under certain conditions gelatine 
and tannin form an insoluble or non-peptisable mixture ;°! but 
we do not know why. The case of chrome tanning is not difficult 
because the chromic oxide would not normally be peptised under 
the conditions of the experiment, and it has been shown that 
there is no necessary change in the gelatine.°? We also cannot 
account at present for the stability or instability of metal sols in 
different organic liquids; in this case as well as in the other 
two, the trouble is in our ignorance of the facts and not in any 
shortcomings of the theory. 
EMULSIONS. 
_By ¥. HATScHEK, Sir John Cass Technical Institute, Lonaon. 
The term Emulsion is used in the following to denote a disperse 
system both phases of which, considered in bulk and at ordinary 
temperature, are liquid. The qualifying clause, although not 
generally stated in such explicit terms, seems desirable as, on one 
hand, the distinction between liquid and solid becomes somewhat 
vague with particles approaching ultra-microscopic dimensions, 
while, on the other, the same system, e.g. rosin in water, may bea 
suspension at lower and an emulsion at higher temperature. 
One characteristic of emulsicns, which distinguishes them sharply 
from systems with solid disperse phase, follows immediately from the 
definition : there is no upper limit to the ratio : Volume of disperse 
phase/Volume of continuous phase. With solid or, better, undeform- 
able particles of disperse phase there is such a limit (apart from the 
ideal case of space-filling polyhedra of equal size) which, for 
spherical particles of uniform radius, is approximately 74/26. 
It is obvious that, with a liquid or deformable disperse phase, 
spherical particles in closest contact do not constitute a limiting case, 
9 Tiebackx, Zeit. Kolloidchemie, 8, 198, 238 (1911). 
91 Wood, Jour. Soc. Chem. Ind., 2'7, 384 (1908). 
Von Schroeder, Zur Kenntniss des Gerbeprozesses. 
Levites. Zeit. Kolloidchemie, 8, 4 (1911). 
*2 Lumiére and Seyewetz, Bull, Soc. Chim., Paris (3) 29, 1077 (1903), 
