The only method of distinguishing between straw and esparto is by examination with the microscope. There is no chemical reagent known which will produce a colour reaction on a paper containing straw that will serve to distinguish it from a paper containing esparto. If such papers are gently heated in a weak solution of aniline sulphate a pink colour is slowly developed, the intensity of which is to some extent a measure of the amount of straw or esparto present. Straw and esparto are usually described in text-books under one heading, partly because the fibres possess strong resemblances in physical and chemical constitution, and partly because the methods of manufacture are identical. At the same time the qualities of the two pulps are so different that they cannot be used indiscriminately, the one for the other. Straw cellulose cannot be utilised in the place of esparto, particularly for light bulky papers. Hence in magazine and book papers containing a fibre which gives a pink coloration with aniline sulphate it is fairly safe to assume that esparto pulp is present. CHAPTER V WOOD PULP AND WOOD PULP PAPERS THE MANUFACTURE OF MECHANICAL WOOD PULP. Wood is converted into pulp suitable for the manufacture of paper by methods which produce two distinct varieties. The first is _mechanical wood pulp_, so called because it is made by a purely mechanical process. The second is termed _chemical wood pulp_ from the fact that the material is submitted to chemical treatment. _Ground Wood and Cellulose._--The two varieties of pulp are sometimes distinguished by the use of the terms ground wood and cellulose. In the former case the description implies a product consisting of pulp obtained by grinding wood into a fibrous condition, while in the second the word suggests a purified chemical product freed from the resinous and non-fibrous constituents found in wood. This is, in fact, the essential difference, for mechanical wood pulp consists of fibres which have been torn away from wood by means of a grindstone; it differs but slightly in chemical composition from the original raw material and contains most of the complex substances natural to wood. Chemical wood pulp, on the other hand, consists of fibre isolated from wood in such a manner that the complex non-fibrous substances are more or less entirely removed. The difference between these two pulps is shown in the following approximate analysis of spruce wood, and of the pulp derived from it. The composition of the mechanical pulp is practically identical with that of the wood itself. COMPOSITION OF SPRUCE WOOD, AND OF CHEMICAL WOOD PULP (SPRUCE).