Determination 
of osmotic 
pressure 
446 EEPORT OP CHEMICAL LABOBATOHY 
character of the gums, the total quantity of each metal (and also their relative proportions) 
remains much the same throughout; and the differences may perhaps be accounted for by 
the presence of dirt or soil which became accidentally mixed with the gum during the time 
of collecting. 
A purely physical method of examining gums which does not appear to have been 
employed hitherto is the determination of the osmotic pressure of a solution of the gum. 
The type of osmometer which I have employed for this is that described by Moore and 
Eoaf.' It is specially adapted for the rapid diffusion of any crystalloids which may be in 
solution through the parchment-iraper membrane. Moore and Eoaf by this method have 
completely disproved the theory that colloids possess no osmotic pressure. Among other 
colloids employed was gum-arabic, a 6 per cent, solution of which gave an osmotic 
pressure of 184 mm. of mercury after five days; another sample gave a pressure of 
142 mm. Gum tragacanth gave practically no osmotic pressure, but only a very dilute 
solution of this gum can be obtained. 
In my experiments the final reading of the manometer was always made after bringing 
the temperature of the solution to the original temperature, thus avoiding any slight error 
due to expansion or contraction of the solution inside the osmometer. The instrument was 
not disconnected until the pressure had been stationary for at least forty-eight hours. In 
all cases a solution containing six grammes of dry gum in 100 c.c. was employed. 
Oum 
Osmotic pressure in 
mm. of mercury 
Temperature 
.30 middle-sized trees, 1st eollcction. 
1.52 
32-5'’ C. 
„ „ 2nd ,, . 
1G4 
.31-5 
„ „ .3id „ . 
1G4 
.32 
„ „ -ttli 
1G7 
.32 
„ „ .. . 
170 
32 
„ >, Ghi „ . 
165 
.32 
Kordofan “hlo.aehed” gum . ... . 
no 
2.3 
„ selected strong gum . ... . 
127 
2.3 
Gum yielding ropy solution—A 
140 
21 
,> » „ K . 
141 
22 
„ „ „ C . 
118 
19-5 
Kordofan gum, hard tears, yielding a weak solution 
102 
27-5 
„ „ December, 1907 . 
125 
32-5 
Senegal gum, (frossc hJanchr ... 
114 
16-5 
These results, while they may have been affected to a slight extent by the tem])erature, 
show that there is a very marked difference in the osmotic pressure of Hashab gum. On 
the whole the tendency seems to be for the more viscous gums to have a higher pressure 
than the weaker varieties. Unfortunately 1 was unable to test the osmotic pressure of 
any samples of gum within say a month of their being collected. Dr. Beam has shown 
Jiio-r.Iu'iiiicitl Journal^ Vol. II., j). 34. 
