*--The simplest way of determining what
elements make up the different nutrients is by heating them and studying
the products of decomposition, as follows:
_To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen._--Place
one half teaspoonful of powdered starch in a test tube and heat strongly.
Observe that _water_ condenses on the sides of the tube and that a black,
charred mass remains behind. The black mass consists mainly of _carbon_.
The water is composed of hydrogen and oxygen. These three elements are
thus shown to be present in the starch. The experiment may be repeated,
using sugar instead of starch.
_To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and
Sulphur._--Place in a test tube some finely divided proteid which has been
thoroughly dried (dried beef or the lean of hard cured bacon). Heat
strongly in the hood of a chemical laboratory or some other place where
the odors do not get into the room. First hold in the escaping gases a wet
strip of red litmus paper. This will be turned blue, showing _ammonia_
(NH3) to be escaping. Next hold in the mouth of the tube a strip of a
paper wet with a solution of lead nitrate. This is turned black or brown
on account of _hydrogen sulphide_(H2S) which is being driven off.
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