On the occurrence of Mannite in Laminaria Saccharina. 41 
inte a tank; and be subjected to such treatment, from ad- 
mixture or otherwise, as has been found by experience likely 
to render it more efficient. These results, moreover, I need 
hardly remark, are perfectly in accordance with the expe- 
rience of intelligent farmers, in many instances on record, of 
the extraordinary fertilizing effects of irrigation with waters 
—the washings and drainage of the farm-yard and dung-heap. 
THE Oaks, AMBLESIDE, 
Oct. 12. 1844. 
On the Occurrence of Mannite in the Laminaria saccharina 
and other Sea-weeds ; also in Mushrooms. 
It appears, from the experiments of Dr John Stenhouse, as con- 
tained in the Philosophical Magazine for October 1844, that the 
Laminaria saccharina contains 12.15 per cent. of Mannite. 
Mannite may be easily distinguished from cane-sugar by the fol- 
lowing test :—If a little strong sulphuric acid is poured upon the 
mannite, and a gentle heat is applied, the mannite dissolves without 
being in the least discoloured, and gives a transparent solution. If 
the heat is much increased the liquid becomes of a deep-brown colour, 
but does not lose its transparency. When cane-sugar, on the con- 
trary, is gently heated with sulphuric acid, it is, as is well known, 
immediately charred with evolution of sulphurous acid gas. From 
grape-sugar mannite may be likewise easily distinguished. If man- 
nite is boiled with a strong solution of potash or soda, it dissolves 
without any change of colour; while grape-sugar, when similarly 
treated, acquires a deep-brown colour, When heated with a solu- 
tion of potash and some sulphate of copper, mannite completely pre- 
vents the precipitation of the oxide of copper; while grape-sugar 
causes the immediate precipitation of the red oxide of copper. 
Besides mannite, the Laminaria saccharina, in common with most 
of the other sea-weed, contains a great deal of peculiar mucilage, 
which, when dried, has a deep-reddish colour. It differs, however, 
from ordinary gum ; for, when digested with nitric acid, it yields 
oxalic, but neither mucic nor saccharic acids. I intend subjecting this 
substance to more minute examination. 
Laminaria digitata. —Besides the L. saccharina, I have also exa- 
mined some of the other sea-weeds for mannite, and among others the 
L. digitata or common tangle. The aqueous solution of this sea- 
weed is also reddish-brown, and when evaporated, it yields a similar 
mucilage with the L. saccharina, but in much smaller quantity. 
The L. digitata also contains a considerable quantity of mannite, 
