174 PRACTICAL LESSONS IN SCIENCE. 



and is the most abundant constituent; the lignine is the material 

 lining the cells, and is more abundant in the harder varieties of 

 woods. Woods differ widely in their physical properties, but are 

 quite similar in their chemical composition. Beech, birch, oak, 

 aspen and willow each contain about the same amount of car- 

 bon. Aspen 49.26 per cent., birch 50.29 percent., and the others 

 between these amounts varying less than 1 per cent. Of oxygen 

 willow contains 39.38 per cent., beech 42.36 per cent., the others 

 between, varying less than 4 per cent. The hydrogen, sulphur, 

 nitrogen and ash are also nearly the same in each. Cellulose is 

 nearly pure in cotton, linen and the best kinds of paper. If dry 

 white paper, pure cellulose, be drawn through a cooled mixture 

 of two parts of strong sulphuric acid and one part of water, and 

 then thoroughly washed, it is changed into a semi-transparent 

 substance called vegetable parchment; when dried it is found 

 that there has been no alteration in weight or composition, but 

 it is at least five times as strong as before, and is waterproof. 

 This change is supposed to be due to change in the molecule. 

 Cellulose treated with a mixture of nitric and sulphuric acids is 

 converted into nitrocellulose or gun cotton. Gun cotton dis- 

 solved in a mixture of alcohol and ether is the collodion much 

 used by photographers. 



