160 PHYSIOLOGICAL VIEWS. 



juice, or any other acid, will turn green to a brown or yellow 

 colour. 



The second class of proximate principles consists of sub- 

 stances which, like the first class, are formed of carbon, hy- 

 drogen, and oxygen ; but to these is added nitrogen. In this 

 class we find, 



Opium, a narcotic principle, extracted from the poppy. It 

 is soluble in alcohol, slightly soluble in water. 



Hematine; this is the colouring principle, from the cam- 

 peachy wood. 



Indigo, a colouring substance, obtained from several species 

 of Indigofera, or the indigo plant. 



Gluten, is extracted from the cotyledons of the seeds of legu- 

 minous plants, as peas, beans ; and from the albu tien of wheat, 

 rye, &c. It is obtained by separating it from the starch ; flour 

 owes much of its nourishing properties to gluten ; which in 

 some respects is analogous to animal principles, and like them 

 is subject to putrefaction. 



Jelly p , is the thickened juice of succulent fruits ; as currants, 

 quinces, and apples ; it is soluble in hot water, though scarcely 

 so in cold ; when heated, it loses its jelly-like form, which is 

 that of a coagulated mass, between a liquid and a solid, suscepti- 

 ble of a tremulous motion ; by long boiling, the juice loses the 

 property which gives to jelly its peculiar appearance. Many 

 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 with 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. 



* Iron is supposed to be combined with the oxygen of the acid. 



What new element is found in the second class of proximate principles ? 

 What substances are found in this class? Cause of the red colour of fruit 

 Of the various hues of the petals of flowers. , - 



