74 REPORTS ON THE STATE OF SCIENCE.—1917. 
concentration of 2 per cent. each of gelatin and gum, at a temperature of 
80° C., there is no sign of flocculation. If, however, the proportion of gum 
is increased, a turbidity appears which again disappears on increasing the 
proportion of gelatin and gum. Strong solutions of gelatin and gum, e.g., 
50 c.c. of 50 per cent. solutions of each, yielded no precipitate with N/10 
HCl, or stronger solutions of the acid. When sulphuric acid is added to a 
solution containing 0-5 per cent. of gelatin and 0-7 per cent. of gum, a 
turbidity occurs when the acidity of the solution is =0-003N. <A solution of 
400 c.c. of a 0-4 per cent. solution of gelatin and 100 c.c. of a 2 per cent. 
solution of gum is coagulated by the addition of 500 c.c. of 6 per cent. acetic 
acid. The coagulum consists of an adsorption compound containing 
gelatin, gum, and acid, the gelatin being enclosed by the gum, which latter 
adsorbs the acid. On washing or drying at 100° C. the acid is removed. 
Two layers are formed, the lower one behaving in a similar way to globulin 
or casein in presence of electrolytes, the view being expressed that casein 
and other proteids may also be adsorption compounds, the constituents 
resembling one another both in properties and in composition. In the 
above reaction the acid may be replaced by salts, the most active being 
tartrates, citrates, phosphates, and acetates, followed by sulphates, 
chlorides, Lromides, and iodides, the activity of the cations being in the 
following order, Na)K)NH,)Mg)Zn). Salts cause the coagulum produced 
by acids to swell, the activity of the anions in this respect being in the 
order I)Br)Cl)S0,)CO,) and the cations K)Na) alkalies) alkaline earths). 
When, however, concentrated solutions of salts or those of 3 gram molecule 
strength or less are employed, the already swollen transparent gel becomes 
again opaque. The tendency of a saline solution is to remove the acid, 
while in presence of salts greater concentration of acid is required to 
produce a given effect. 
Gum arabic solutions show a very high osmotic pressure, which remains 
constant over a long period. Thus Pfeffer, using a copper ferrocyanide 
membrane, records a pressure of 7-2 ¢.m. of mercury with a 1 per cent. 
solution and 120-4 ¢.m. with an 18 per cent. solution (Ostwald, ‘ Solutions,’ 
p. 94), while Moore and Roaf (‘ Biochem. Jour.’ 2, 39) and Edie (‘ 4th 
Report Wellcome Research Laboratories, Khartoum’), using a parchment 
paper diaphragm, found for 6 per cent. solutions pressures of 114-170 m.m., 
and for a 10 per cent. solution 276 mm. The reason for these high 
pressures, which remain almost constant after reaching a maximum, has 
not yet been explained ; it may be due to the slow diffusion of the elec- 
trolytes associated with the carbohydrates, or it may be a swelling or 
imbibition pressure (pseudo-osmotic). 
In testing gum arabic for commercial purposes the viscosity is the most 
important property (R. Hefelmann, ‘ Zeitschr. Offentl. Chem.’ 7, 195-198). 
C. Fromm (° Zeitschr. Annal. Chem.’ 1901, 40, 143-168) soaks strips of paper 
in solutions of the gunis to be tested and notes the increase in weight and 
tensile strength of the paper which serves as a measure of the value of the 
gums for adhesive purposes. The latter author also states that on keeping 
gums a gradual change takes place in them which in the trade is known 
as ‘ripening’; this change leads to a decrease in strength, viscosity, and 
acidity, at the same time there is an increase in the proportion of gelatinised 
insoluble gum, 
The problem of the effect of colloids upon the precipitation of mineral 
matter is a very interesting one, which would well repay further study, not 
