According to the view of Cross and Bevan, cellulose conforms in respect of its ultimate constitutional groups to the general features of the simple carbohydrates, but differs from them by reason of a special molecular configuration resulting in a suppression of the activity of constituent groups in certain respects, and an increase in activity of others.[221] =271. Fiber and Cellulose.=—The carbohydrates of a plant insoluble in water are not composed exclusively of starch. There are, in addition to starch, pentosan fibers yielding pentose sugars on hydrolysis and furfuraldehyd on distillation with a strong acid. The quantitive methods for estimating the pentosan bodies are given in paragraphs =150-157=. The method to be preferred is that of Krug (=155=). In the estimation of cattlefoods and of plant substances in general the residue insoluble in dilute boiling acid and alkali is called crude or indigestible fiber. The principle on which the determination depends rests on the assumption that all the protein, starch and other digestible carbohydrates will be removed from the sample by successive digestion at a boiling temperature with acid and alkali solutions of a given strength. It is evident that the complex body obtained by the treatment outlined above is not in any sense a definite chemical compound, but it may be considered as being composed partly of cellulose. =272. Official Method of Determining Crude Fiber.=—The method of estimating crude fiber, adopted by the Association of Official Agricultural Chemists, is as follows:[222] Extract two grams of the substance with ordinary ether, at least almost completely, or use the residue from the determination of the ether extract. To this residue, in a half liter flask, add 200 cubic centimeters of boiling 1.25 per cent sulfuric acid; connect the flask with an inverted condenser, the tube of which passes only a short distance beyond the rubber stopper into the flask. Boil at once, and continue the boiling for thirty minutes. A blast of air conducted into the flask may serve to reduce the frothing of the liquid. Filter, wash thoroughly with boiling water until the washings are no longer acid, rinse the substance back into the same flask with 200 cubic centimeters of a boiling 1.25 per cent solution of sodium hydroxid, free or nearly free of sodium carbonate, boil at once and continue the boiling for thirty minutes in the same manner as directed above for the treatment with acid. Filter into a gooch, and wash with boiling water until the washings are neutral, dry at 110°, weigh and incinerate completely. The loss of weight is crude fiber. The filter used for the first filtration may be linen, one of the forms of glass wool or asbestos filters, or any other form that secures clear and reasonably rapid filtration. The solutions of sulfuric acid and sodium hydroxid are to be made up of the specified strength, determined accurately by titration and not merely from specific gravity.