264 Dr. Stenhouse on the occurrence of Mannite in the 



derable. One thousand grains of the dried sea-weed, when 

 treated in the way already described, first with water and then 

 with alcoho],gavel2r5 grains =12*15 per cent. The specimen 

 operated on had been kept for six months, and was covered 

 with an incrustation in long slender needles. I also examined 

 a quantity of the fresh sea-weed, and found it to contain as 

 much mannite as the older specimen. It appears, therefore, 

 that the epithet saccharina^ as applied to this species of La- 

 minaria, is not so inappropriate as has been often supposed. 



Mannite may be easily distinguished from cane-sugar by 

 the following 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 trans- 

 parency. When cane-sugar, on the contrary, 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 mannite is 

 boiled with a strong solution of potash or soda, it dissolves 

 without any change of colour, while grape-sugar when simi- 

 larly treated acquires a deep brown colour. When heated with 

 a solution of potash and some sulphate of copper, mannite 

 completely prevents the precipitation of the oxide of copper, 

 while grape-sugar causes the immediate precipitation of the 

 red oxide of copper. 



Besides mannite, theZ*. saccharina^ in common with most of 

 the other sea-weeds, contains a great deal of a peculiar muci- 

 lage, 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 or saccharic acids. I 

 intend subjecting this substance to more minute examination. 



L. digitata. — Besides the L, saccharina I have also ex- 

 amined some of the other sea-weeds for mannite, and among 

 others the L. digitata or common tangle. Th^ aqueous solu- 

 tion of this sea-weed is also reddish brown, and when evapo- 

 rated it yields a similar mucilage with the L. saccharina, but 

 in much smaller quantity. The L, digitata also contains a 

 considerable quantity of mannite, though I should think 

 scarcely half as much as what exists in the L. saccharina. 



Halydris siliquosa. — The next sea-weed examined was the 

 Halydris siliquosa. With hot water it forms a very dark- 

 coloured solution of a bitter and slightly astringent taste. The 

 quantity of mannite contained in it is very great, amounting I 

 sjiould think to between 5 and 6 per cent. As already men- 



