Ixvi _ Historical Sketch of the Physical Sciences, 1818. 
substance he considers as very analogous to suga ; the second 
as peculiar. (Annals va Philosophy, xii. 153.) If these con- 
clusions were accurate, Vauquelin’s results, that manna is inca- 
pable of undergoing the vinous fermentation, could not be 
true. 
3. Starch—The blue colour produced upon starch by the 
action of iodine is now well known, and this re-agent is ac- 
cordingly frequently employed to detect the presence of starch 
in vegetable bodies. M. Vincent has discovered that prussian 
blue is not without its action on starch. If four parts of starch 
and one of prussian blue be triturated in a mortar, and then 
boiled in a considerable quantity of water, it becomes green and 
then brown, and does not recover its blue colour, though treated 
with an acid. The liquor forms a fine prussian blue when mixed 
with equal volumes of sulphate of iron and solution of chlorine. 
It would appear that in this process the starch is altered in its 
nature, and converted into a kind of gum.—(See Amnals of Phi+ 
losophy, xiii. 68.) 
4. Colouring Matters of Vegetables —Respecting the nature 
of the substances to which the different colours in the vegetable 
kingdom are owing, very little satisfactory information has yet 
been acquired by chemists. Many of them are of so fugitive a 
nature, as to baffle every attempt to obtain them in an isolated 
state, while others, which are of a more permanent nature, can- 
hot be easily freed from the various foreign bodies with which 
they are in combination. Mr. Smithson has favoured the world 
with a number of facts respecting these colouring matters, which 
though imperfect and isolated will not be without their utility. 
The infusion of turnsol (ditmus) contains no alkali, lime, nor 
acid; and its natural colour is blue. When the colouring mat- 
ter of turnsol is burnt, it leaves a saline matter, which, with 
nitric acid, forms nitrate of potash. Mr. Smithson suspects 
that this colouring matter, like ulmin, is a compound of a vege- 
table substance and potash. The assertion of Fourcroy that the 
natural colour of turnsol is red, and that it contains carbonate 
of soda, he finds without foundation. ' 
The colouring matter of the violet is blue, but is changed by 
acids to red. Mr. Smithson informs us that the same colouring 
matter exists in the petals of the red rose, in the petals of red 
cloves, in the red lips of the petals of the common daisy, of 
the blue hyacinth, hollyhock, lavender, the inner leaves of the 
artichoke, and numerous other flowers. It colours the skin of 
several plants, of the scarlet geranium, and pomegranate tree. 
‘Red cabbage, and-the rind of the long radish, are coloured by 
the same principle. Mr. Smithson conceives that the acid 
which reddens the-radish is the carbonic. 
-Mr. Smithson gives us hkewise a series of experiments on 
‘sugar loaf paper, on the juice of the black mulberry, the petals 
