I4G CHEMICAL COMPOSITION OF THE SAP. 
colouring principles have never been separated from the sub- 
stances to which they are united ; as those of saffron, logwood, 
&c. 
it has already been suggested, that the red colour of fruits, 
arises from the combination of an acid, with a blue colouring 
principle ; you know that the effect of mixing any acid with an 
infusion of blue violets, or any vegetable blue, is to give a red 
tinge, varying in shade from a purple red to a brilliant scarlet, in 
proportion to the quantity of acid. 
it has also, upon the same principle, been supposed that the 
purple, red, and blue colouring of the petals of flowers is owing 
to different proportions of acid ; this may explain the change 
of colour which appears in some flowers, which pass from blue 
to red ; as the changeable hydrangea. This change may be 
attributed to increase of acid combining Avith the blue colouring 
principle.* Some red flowers become blue; they are in this 
case supposed to have parted with some portion of the acid, 
which was united with their colouring principle. 
Chemical composition of the Sap. 
The sap is a transparent, colourless fluid, imbibed by the veg- 
etable from the earth and air ; or more property, from the Avater 
existing in them, Avhich holds in solution oxygen, hydrogen, 
carbon, nitrogen, earths, mineral salts, and animal and vegeta- 
ble matter. We might suppose, that being derived from the 
same source, the sap in all vegetables Avould be alike, but it is 
never obtained pure ; it is mingled with the proximate princi- 
ples , or proper juices, and thus differs in different species of 
vegetables ; Avater, hoAvever, constitutes the principal part in all. 
The folloAving result of the analysis of the sap of some veg- 
etables has been offered by a French chemist. | 
Sap of the elm, (Ulmds campestris ,) Avater, volatile matter, 
acetate of potash, carbonate of lime, vegetable matter, sulphate 
of potash. 
Sap of the beech, (Fagcs sylrcitica,) water, acetate of lime, 
with excess of acid, acetate of potash, gallic acid, tannin, mu- 
cous extract, acetate of alumine. 
Sap of the Horse Chesnut, (aEfcclus hippocastanum,) Avater, 
extractive mucous matter, nitre, acetate of potash, and carbonate 
of lime. 
These feAv examples of the decomposition of vegetable prin- 
* Iron is supposed to be combined with the oxygen of the acid. 
+ Vauquclin. 
Cause of the red colour of fruit — 
