The woods used for this purpose are principally poplar and spruce, and there are three classes of the wood pulp:  (1) mechanical wood, (2) soda process wood, and (3) sulphite wood pulp.  The first method was invented in Germany in 1844.  The logs are hewn in the forest, roughly barked, and shipped to the factory, where the first operation is to cut them up by steam saws into blocks about two feet in length.  Any bark that may still cling to the log is removed by a rapidly revolving corrugated wheel of steel, while the larger blocks are split by a steam splitter.  The next stage of their journey takes these blocks to a great millstone set perpendicularly instead of horizontally.  Here a very strong and ingenious machine receives one block at a time, and with an automatically elastic pressure holds it sidewise against the millstone, which, like the mills of the gods, “grinds exceeding fine,” and with the aid of constantly flowing water rapidly reduces these blocks to a pulpy form.  This pulp is carried into tanks, from which it is passed between rollers, which leave it in thick, damp sheets, which are folded up evenly for shipment, or for storage for future use.  If a paper-mill is operated in connection with the pulp-mill, the wood pulp is not necessarily rolled out in sheets, but is pumped directly from the tanks to the beaters.

In the preparation of pulp by the other processes, the blocks are first thrown into a chipping machine with great wheels, the short, slanting knives of which quickly cut the blocks into small chips.

In the soda process, invented by M. Meliner in France in 1865, the chips from spruce and poplar logs are boiled under pressure in a strong solution of caustic soda.

When sulphite wood pulp is to be prepared, the chips are conveyed from the chipper into hoppers in the upper part of the building.  Here they are thrown into great upright iron boilers or digesters charged with lime-water and fed with the fumes of sulphur which is burned for the purpose in a furnace adjoining the building and which thus forms acid sulphide of lime.  The sulphite process was originally invented by a celebrated Philadelphia chemist, but was perfected in Europe.

The “cooking,” or boiling, to which the wood is subjected in both the soda and sulphite processes, effects a complete separation of all resinous and foreign substances from the fine and true cell tissue, or cellulose, which is left a pure fiber, ready for use as described.  In the case of all fibers, whether rag or wood, painstaking work counts, and the excellence of the paper is largely dependent upon the time and care given to the reduction of the pulp from the original raw material.