188 



substances are which vary so greatly in solubility, in what materials 

 and under what conditions they occur, and how to classify, separate, 

 and determine them. 



The nutritive values of the materials which are more properly grouped 

 as fats, i. e. the glycerides of the fatty acids and the fatty acids them- 

 selves, are pretty well understood, though further investigation of the 

 molecular constitution of some, and of the potential energy of all, is 

 needed. According to the present outlook it seems probable that, 

 although the waxes and perhaps the lecithins may be classed M'ith the 

 fats in estimations of the nutritive values, a separate classification of 

 some or all of the others will be necessary ; and it is clear that a more 

 definite knowledge of the chemical constitution of all the materials, 

 otherthan the neutral fats and fatty acids, is indispensable to any correct 

 estimate of their values for nutriment. 



Nitrogen-free extract — Carbohydrates — Crude fiber. — For purposes of 

 analysis and determination of nutritive values it will probably prove 

 desirable to reclassify these substances on the basis of their solubilities, 

 not only in water and dilute acid and alkali, but also by such means as 

 treatment with steam at high pressure, and with digestive ferments. 

 The sugars, starches, so-called insoluble carbohydrates, indeed all the 

 materials which we now crudely group together as non-nitrogenous ex- 

 tractives, and those which we call crude fiber demand study. It is 

 essential to investigate carefully the carbohj^d rates which occur in dif- 

 fei-ent plants and at diiferent periods of growth and to learn their be- 

 havior with solvents and their methods of separation. It is also impor- 

 tant to learn more about their digestibility and the ways in which they 

 are utilized in the body. Much of the information must be gained by 

 the methods of vegetable physiology and histology, rather than by 

 purely chemical methods. 



Of the nutritive values of some of the carbohydrates, especially those 

 which are soluble in water or are rendered soluble by the diastatic fer- 

 ments, we have some definite knowledge. For instance, the ordinary 

 sugars may be reckoned "as completely digestible, and the potential en- 

 ergy of a number of them has already been determined, and others are 

 being studied. The problem here is comparatively simple. With those 

 that are not dissolved by either water or the digestive ferments the case 

 is somewhat more complicated, but it is to be hoped that research of the 

 kinds now being carried on so successfully^ by several chemists in this 

 country and in Europe will bring, before many years, much of the defi- 

 nite information which is needed. 



As regards the bodies which we are wont to classify as amides, ether 

 extract, nitrogen-free extract, and crude fiber, and which we attempt 

 to estimate by treatment with water, ether, dilute acid and alkali, or 

 other solvents, we are coming to appreciate that their solubility is influ- 

 enced not only by the fineness of grinding of the sample, the time and 

 temperature of the extraction, the quantity of the water, the purity 



