398 SCIENCE PROGRESS. 



plex which we designate cellulose. Judged by the chemical 

 criterion the paper-maker's celluloses divide themselves into 

 three well marked groups which we may designate by their 

 typical representatives as the Cotton, Wood and Esparto 

 groups ; and these groups correspond with the classification 

 of the botanist. 



This has only to be pointed out in specific terms to 

 those accustomed to the positive discipline of natural 

 science and they will start at once to inquire whether the 

 sheet they are now handling is of the rag, wood or esparto 

 class, knowing perfectly well that important consequences 

 must follow from fundamental differences of composition, 

 with the added significance of differences of physiological 

 function. Some of these consequences are easily brought 

 to demonstration. Take a " Swedish ' filter paper as a 

 normal standard, serving as basis of comparison, and select 

 a paper of each of the above classes, as used for writing or 

 printing purposes. Such papers it must be premised con- 

 tain a proportion of "sizing" constituents; that is, they 

 consist of the respective celluloses with " organic " colloids, 

 e.g., gelatine or starch, and fatty or resinous bodies preci- 

 pitated into the sheet from alkaline solutions by means of 

 sulphate of alumina. These constituents, jointly and 

 severally, contribute a certain water or ink-resisting quality, 

 which we may define in precise terms as so controlling or 

 limiting the absorption of aqueous solutions that it takes 

 place in the vertical (downward) direction only. It is ne- 

 cessary to mention these qualifying or auxiliary constituents 

 as they contribute directly to the effects we are about to 

 describe. We have asked the reader to " select " papers of 

 the typical classes ; but on the assumption that he may 

 probably be doing this for the first time some directions 

 are required. There we meet a difficulty. It might occur 

 to the layman that he has only to go to " the Shop," there 

 to ask and to get. But stationers are notoriously station- 

 ary. Their classification of papers is strictly limited to 

 such side issues as price, " get up," colour and so forth. 

 The shopman's vision generally is limited by the horizon 

 of values, that simple calculus of differences by which the 



