PROXIMATE PRINCIPLES. 
115 
like milk, but they usually become brown and hard by exposure 
to the air. 
Indian rubber,* or as it is sometimes called, gum elastic, is 
the product of a South American tree, called the Siphonia elas- 
tica, an East Indian plant, the Urceola elastica, and some other 
trees in the equatorial regions ; by exposure to the air it hard- 
ens, becomes brown, and takes the appearance of leather; it 
can neither be dissolved by water nor alcohol. The juice of the 
milk weed is said to be similar to that of the Siphonia elastica, 
and that of other plants from which the Indian rubber is made.f 
The green principle; it is to this principle that all the green 
parts, exposed to light, owe their colour ; it undergoes changes 
in the different states of the plant, in autumn becoming brown 
or yellow. Davy attributes the change of colour, to the formation 
of some acid ; you know that a drop of sour wine, lemon juice, 
or any other acid, will turn green to a brown or yellow colour. 
II. Second class of Proximate Principles. 
Tl te 2d class of proximate elements are formed of carbon, 
hydrogen, and oxygen, which also compose the preceding class ; 
but to this 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 campea- 
chy 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 albumen 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 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, sus- 
ceptible of a tremulous motion ; by long boiling, the juice loses 
the property which gives to jelly its peculiar appearance. Many 
* Caoutchouc. 
t Mr. H. Eaton, 'assistant professor at the Rensselaer Institution, prepared 
a small quantity of the juice of the milk weed (Asclepias) in such a man- 
ner that it could not be distinguished from the imported Indian rubber, 
either in external appearance, or in its properties. 
Second class of proximate principles or elements. 
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