30. Proteids. Let us return now to that other constituent of the wheaten dough, the elastic material which remained after removal of the starch. This is known as gluten1 and is a mixture of several substances which belong to the class known as proteids.2 To this class belong also the substances which form the chief part of our own flesh and blood-and indeed, mainly constitute the living substance of all plants and animals. Hence, proteids must be regarded as the most precious of all food substances. Like the carbohydrates they contain carbon, hydrogen, and oxygen (though in somewhat different proportions), but in addition they always have a certain amount of nitrogen, and usually a little sulphur and phosphorus. The nitrogenous nature of the proteids is made evident by the pungent ammoniacal odor which is given off when any of them are burned,-ammonia being NH3. Although in chemical composition proteids are all very much alike, there are important differences in their solubility-some, like white of egg, dissolving in cold water, while others, such as those of the "wheat gum" are insoluble. Among the latter is a form of proteid called glutin or gliadin which gives to wheat-gluten its wonderful tenacity and elasticity.

1 Glu'ten - L. glutus, tenacious.

2 Pro'te-id - Gr. protos, before.

It is a significant fact that wheat is the only one of the cereals which contains gliadin in any considerable amount, although it should be said that rye contains a closely similar proteid. Macaroni, which owes its consistency chiefly to gliadin, is therefore made only from wheat; and wheaten dough alone possesses just the right tenacity and elasticity for making the lightest, spongiest loaf. The lightest rye bread has wheat flour mixed with it. The fact that wheat contains in largest amount a nutrient with such remarkable properties as gliadin, is the chief reason why this grain was prized above all others in ancient times, and why it has come to be valued more and more highly as civilization has advanced.