Part 1 (1/2)

Researches on Cellulose.

by C. F. Cross and E. J. Bevan.

PREFACE

This volume, which is intended as a supplement to the work which we published in 1895, gives a brief account of researches which have been subsequently published, as well as of certain of our own investigations, the results of which are now for the first time recorded.

We have not attempted to give the subject-matter the form of a connected record. The contributions to the study of 'Cellulose' which are noticed are spread over a large area, are mostly 'sectional' in their aim, and the only cohesion which we can give them is that of cla.s.sifying them according to the plan of our original work. Their subject-matter is reproduced in the form of a _precis_, as much condensed as possible; of the more important papers the original t.i.tle is given. In all cases we have endeavoured to reproduce the Author's main conclusions, and in most cases without comment or criticism.

Specialists will note that the basis of investigation is still in a great measure empirical; and of this the most obvious criterion is the confusion attaching to the use of the very word 'Cellulose.' This is due to various causes, one of which is the curious specialisation of the term in Germany as the equivalent of 'wood cellulose.' The restriction of this general or group term has had an influence even in scientific circles. Another influence preventing the recognition of the obvious and, as we think, inevitable basis of cla.s.sification of the 'celluloses'

is the empiricism of the methods of agricultural chemistry, which as regards cellulose are so far chiefly concerned with its negative characteristics and the a.n.a.lytical determination of the indigestible residue of fodder plants. Physiologists, again, have their own views and methods in dealing with cellulose, and have hitherto had but little regard to the work of the chemist in differentiating and cla.s.sifying the celluloses on a systematic basis. There are many sides to the subject, and it is only by a sustained effort towards centralisation that the general recognition of a systematic basis can be secured.

We may, we hope usefully, direct attention to the conspicuous neglect of the subject in this country. To the matter of the present volume, excluding our own investigations, there are but two contributions from English laboratories. We invite the younger generation of students of chemistry to measure the probability of finding a working career in connection with the cellulose industries. They will not find this invitation in the treatment accorded to the subject in text-books and lectures. It is probable, indeed, that the impression produced by their studies is that the industries in coal-tar products largely exceed in importance those of which the carbohydrates are the basis; whereas the former are quite insignificant by comparison. A little reflection will prove that cellulose, starch, and sugar are of vast industrial moment in the order in which they are mentioned. If it is an open question to what extent science follows industry, or _vice versa_, it is not open to doubt that scientific men, and especially chemists, are called in these days to lead and follow where industrial evolution is most active. There is ample evidence of activity and great expansion in the cellulose industries, especially in those which involve the chemistry of the raw material; and the present volume should serve to show that there is rapid advance in the science of the subject. Hence our appeal to the workers not to neglect those opportunities which belong to the days of small beginnings.

We have especially to acknowledge the services of Mr. J. F. BRIGGS in investigations which are recorded on pp. 34-40 and pp. 125-133 of the text.

CELLULOSE

INTRODUCTION

In the period 1895-1900, which has elapsed since the original publication of our work on 'Cellulose,' there have appeared a large number of publications dealing with special points in the chemistry of cellulose. So large has been the contribution of matter that it has been considered opportune to pa.s.s it under review; and the present volume, taking the form of a supplement to the original work, is designed to incorporate this new matter and bring the subject as a whole to the level to which it is thereby to be raised. Some of our critics in reviewing the original work have p.r.o.nounced it 'inchoate.' For this there are some explanations inherent in the matter itself. It must be remembered that every special province of the science has its systematic beginning, and in that stage of evolution makes a temporary 'law unto itself.' In the absence of a dominating theory or generalisation which, when adopted, gives it an organic connection with the general advance of the science, there is no other course than to cla.s.sify the subject-matter. Thus 'the carbohydrates' may be said to have been in the inchoate condition, qualified by a certain cla.s.sification, prior to the pioneering investigations of Fischer. In attacking the already acc.u.mulated and so far cla.s.sified material from the point of view of a dominating theory, he found not only that the material fell into systematic order and grew rapidly under the stimulus of fruitful investigation, but in turn contributed to the firmer establishment of the theoretical views to which the subject owed its systematic new birth. On the other hand, every chemist knows that it is only the simpler of the carbohydrates which are so individualised as to be connoted by a particular formula in the stereoisomeric system. Leaving the monoses, there is even a doubt as to the const.i.tution of cane sugar; and the elements of uncertainty thicken as we approach the question of the chemical structure of starch. This unique product of plant life has a literature of its own, and how little of this is fully known to what we may term the 'average chemist' is seen by the methods he will employ for its quant.i.tative estimation. In one particular review of our work where we are taken to task for producing 'an aggravating book, inchoate in the highest degree ... disfigured by an obscurity of diction which must materially diminish its usefulness' ['Nature,' 1897, p. 241], the author, who is a well-known and competent critic, makes use of the short expression in regard to the more complex carbohydrates, 'Above cane sugar, higher in the series, all is chaos,' and in reference to starch, 'the subject is still enshrouded in mystery.' This 'material' complexity is at its maximum with the most complex members of the series, which are the celluloses, and we think accounts in part for the impatience of our critic. 'Obscurity of diction' is a personal quant.i.ty, and we must leave that criticism to the fates. We find also that many workers whose publications we notice in this present volume quite ignore the _plan_ of the work, though they make use of its matter. We think it necessary to restate this plan, which, we are satisfied, is systematic, and, in fact, inevitable. Cellulose is in the first instance a _structure_, and the anatomical relations.h.i.+ps supply a certain basis of cla.s.sification. Next, it is known to us and is defined by the negative characteristics of resistance to hydrolytic actions and oxidations. These are dealt with in the order of their intensity. Next we have the more positive definition by ultimate products of hydrolysis, so far as they are known, which discloses more particularly the presence of a greater or less proportion of furfural-yielding groups. Putting all these together as criteria of function and composition we find they supply common or general dividing lines, within which groups of these products are contained. The cla.s.sification is natural, and in that sense inevitable; and it not only groups the physiological and chemical facts, but the industrial also. We do not propose to argue the question whether the latter adds any cogency to a scientific scheme. We are satisfied that it does, and we do not find any necessity to exclude a particular set of phenomena from consideration, because they involve 'commercial' factors. We have dealt with this cla.s.sification in the original work (p. 78), and we discuss its essential basis in the present volume (p. 28) in connection with the definition of a 'normal' cellulose. But the 'normal' cellulose is not the only cellulose, any more than a primary alcohol or an aliphatic alcohol are the only alcohols. This point is confused or ignored in several of the recent contributions of investigators. It will suffice to cite one of these in ill.u.s.tration. On p. 16 we give an account of an investigation of the several methods of estimating cellulose, which is full of valuable and interesting matter. The purpose of the author's elaborate comparative study is to decide which has the strongest claims to be regarded as the 'standard' method. They appear to have a preference for the method of Lange--viz. that of heating at high temperatures (180) with alkaline hydrates, but the investigation shows that (as we had definitely stated in our original work, p. 214) this is subject to large and variable errors. The adverse judgment of the authors, we may point out, is entirely determined on the question of aggregate weight or yield, and without reference to the ultimate composition or const.i.tution of the final product. None of the available criteria are applied to the product to determine whether it is a cellulose (anhydride) or a hydrate or a hydrolysed product. After these alkali-fusion processes the method of chlorination is experimentally reviewed and dismissed for the reason that the product retains furfural-yielding groups, which is, from our point of view, a particular recommendation, i.e. is evidence of the selective action of the chlorine and subsequent hydrolysis upon the lignone group. As a matter of fact it is the only method yet available for isolating the cellulose from a lignocellulose by a treatment which is quant.i.tatively to be accounted for in every detail of the reactions. It does not yield a 'normal'

cellulose, and this is the expression which, in our opinion, the authors should have used. It should have been pointed out, moreover, that, as the cellulose is separated from actual condensed combination with the lignone groups, it may be expected to be obtained in a hydrated form, and also not as a h.o.m.ogeneous substance like the normal cotton cellulose. The product is a cellulose of the second group of the cla.s.sification. Another point in this investigation which we must criticise is the ultimate selection of the Schulze method of prolonged maceration with nitric acid and a chlorate, followed by suitable hydrolysis of the non-cellulose derivatives to soluble products. Apart from its exceptional inconvenience, rendering it quite impracticable in laboratories which are concerned with the valuation of cellulosic raw materials for industrial purposes, the attack of the reagent is complex and ill-defined. This criticism we would make general by pointing out that such processes quite ignore the specific characteristics of the non-cellulose components of the compound celluloses. The second division of the plan of our work was to define these const.i.tuents by bringing together all that had been established concerning them. These groups are widely divergent in chemical character, as are the compound celluloses in function in the plant. Consequently there is for each a special method of attack, and it is a reversion to pure empiricism to expect any one treatment to act equally on the pectocelluloses, lignocelluloses, and cutocelluloses. Processes of isolating cellulose are really more strictly defined as methods of selective and regulated attack of the groups with which they occur, combined or mixed. A chemist familiar with such types as rhea or ramie (pectocellulose), jute (lignocellulose), and raffia (cutocellulose) knows exactly the specific treatment to apply to each for isolating the cellulose, and must view with some surprise the appearance at this date of such 'universal prescriptions' as the process in question.

The third division of our plan of arrangement comprised the synthetical derivatives of the celluloses, the sulphocarbonates first, as peculiarly characteristic, and then the esters, chiefly the acetates, benzoates, and nitrates. To these, investigators appear to have devoted but little attention, and the contribution of new matter in the present volume is mainly the result of our own researches. It will appear from this work that an exhaustive study of the cellulose esters promises to a.s.sist very definitely in the study of const.i.tutional problems.

This brings us to the fourth and, to the theoretical chemist, the most important aspect of the subject, the problem of the actual molecular structure of the celluloses and compound celluloses. It is herein we are of opinion that the subject makes a 'law unto itself.' If the const.i.tution of starch is shrouded in mystery and can only be vaguely expressed by generalising a complex ma.s.s of statistics of its successive hydrolyses, we can only still more vaguely guess at the distance which separates us from a mental picture of the cellulose unit. We endeavour to show by our later investigations that this problem merges into that of the actual structure of cellulose in the ma.s.s. It is definitely ascertained that a change in the molecule, or reacting unit, of a cellulose, proportionately affects the structural properties of the derived compounds, both sulphocarbonates and esters. This is at least an indication that the properties of the visible aggregates are directly related to the actual configuration of the chemical units. But it appears that we are barred from the present discussion of such a problem in absence of any theory of the solid state generally, but more particularly of those forms of matter which are grouped together as 'colloids.'

Cellulose is distinguished by its inherent constructive functions, and these functions take effect in the plastic or colloidal condition of the substance. These properties are equally conspicuous in the synthetical derivatives of the compound. Without reference, therefore, to further speculations, and not deterred by any apparent hopelessness of solving so large a problem, it is clear that we have to exhaust this field by exact measurements of all the constants which can be reduced to numerical expression. It is most likely that the issue may conflict with some of our current views of the molecular state which are largely drawn from a study of the relatively dissociated forms of matter. But such conflicts are only those of enlargement, and we antic.i.p.ate that all chemists look for an enlargement of the molecular horizon precisely in those regions where the forces of cell-life manifest themselves.

The _cellulose group_ has been further differentiated by later investigations. The fibrous celluloses of which the typical members receive important industrial applications, graduate by insensible stages into the hemicelluloses which may be regarded as a well-established sub-group. In considering their morphological and functional relations.h.i.+ps it is evident that the graduation accords with their structure and the less permanent functions which they fulfil. They are aggregates of monoses of the various types, chiefly mannose, galactose, dextrose, &c., so far as they have been investigated.

Closely connected with this group are the const.i.tuents of the tissues of fungi. The recent researches of Winterstein and Gilson, which are noted in this present volume, have established definitely that they contain a nitrogenous group in intimate combination with a carbohydrate complex.

This group is closely related to chitin, yielding glucosamin and acetic acid as products of ultimate hydrolysis. Special interest attaches to these residues, as they are in a sense intermediate products between the great groups of the carbohydrates and proteids (E. Fischer, Ber. 19, 1920), and their further investigation by physiological methods may be expected to disclose a genetic connection.

The _lignocelluloses_ have been further investigated. Certain new types have been added, notably a soluble or 'pectic' form isolated from the juice of the white currant (p. 152), and the pith-like wood of the aeschynomene (p. 135).

Further researches on the typical fibrous lignocellulose have given us a basis for correcting some of the conclusions recorded in our original work, and a study of the esters has thrown some light on the const.i.tution of the complex (p. 130).

Of importance also is the identification of the hydroxyfurfurals as const.i.tuents of the lignocelluloses generally, and the proof that the characteristic colour-reactions with phenols (phloroglucinol) may be ascribed to the presence of these compounds (p. 116).

The _pectocelluloses_ have not been the subject of systematic chemical investigation, but the researches of Gilson ('La Cristallisation de la Cellulose et la Composition Chimique de la Membrane Cellulaire Vegetale,' 'La Revue,' 'La Cellule,' i. ix.) are an important contribution to the natural history of cellulose, especially in relation to the 'pectic' const.i.tuents of the parenchymatous celluloses.

Indirectly also the researches of Tollens on the 'pectins' have contributed to the subject in correcting some of the views which have had a text-book currency for a long period. These are dealt with on p.

151. The results establish that the pectins are rather the soluble hydrated form of cellulosic aggregates in which acid groups may be represented; but such groups are not to be regarded as essentially characteristic of this cla.s.s of compounds.