THE USES OF WOOD. 221 



pulp is made in two ways: First, chemical; second, 

 mechanical. Chemical pulp is made by treating the tissues 

 of the wood with chemicals which dissolve out the lignin 

 substances until only the cellulose or pulp is left. There 

 are several processes used, one of which is by boiling the 

 wood in Glauber salts for seven hours, after which the 

 pulp is washed clean and bleached with chloride of lime 

 until quite white. This process is more expensive than 

 the sulphite process, in which sulphurous acid is used, 

 but is said to give a better product. All kinds of wood 

 may be converted into chemical pulp, but very dark 

 colored and very resinous kinds are usually avoided. 

 Ordinary paper pulp is made by grinding Spruce wood 

 on large stones, against which it is pressed by hydraulic 

 power. For the manufacture of very tough paper and 

 of leather board it is customary to boil the wood a long 

 time before it is ground. Wood that is cooked a long 

 time before grinding makes a dark colored but very tough 

 product. In the manufacture of ordinary printing paper, 

 the wood is put at once upon the stone, and the pulp 

 from it is rolled into paper. It is customary to mix a 

 certain per cent, of chemically prepared paper pulp with 

 ordinary wood pulp in order to improve its strength. 

 Wood pulp is not only manufactured into paper, but 

 boards, buckets, car wheels, and a thousand other things, 

 are made from it. Its use is almost unlimited. 



By the Destructive Distillation of Wood (all kinds 

 of wood, all sizes logs, refuse, sawdust) we obtain char- 

 coal, vinegar, alcohol, creosote, gas, tar; pyroligneous, 

 oxalic, acetic, and other acids; acetone, paraffin, naph- 

 thalin; lampblack and other products. From the bark 

 of Oak, Hemlock, Chestnut, and other trees the tannic 

 acid used* in tanning leather is obtained. 



As we have digressed slightly by including bark under 

 the term "wood/! we may go further, and take in also 



