Page:Life in Motion.djvu/158

138 as I have said, represents the first essential constituent of a diet, proteid or nitrogenous matter.

Now let us examine the whey. It is sweet from containing a kind of sugar called milk-sugar. These sugar substances, when heated with an alkaline solution of a salt of copper, have the property of taking oxygen from the copper compound, changing it to one that is insoluble; and in effecting this change they also cause a change of colour. Thus you see, when I add to this solution of grape-sugar a blue solution of a copper salt (called Fehling's solution, an alkaline tartrate of copper) and heat it, the blue colour gradually disappears and a reddish substance, an oxide of copper (cuprous oxide) falls to the bottom of the glass. Applying the same test to the whey, we find proof of the existence of sugar in it. Now sugar is a compound of carbon, hydrogen, and oxygen. It contains no nitrogen, and hence it is called non-nitrogenous, to distinguish it from the nitrogenous or proteid group of bodies already referred to. As the oxygen and hydrogen in sugar are in the proportion by volume of one of oxygen to two of