480 SMITH'S INTERMEDIATE CHEMISTRY 



its solution in water has an intensely sweet taste, hence it is used 

 as a substitute for sugar in war times and in cases of diabetes. 

 It has no food value, however, and hence cannot replace sugar 

 in nutrition. 



Plastics. These are substances, like celluloid, which can 

 be moulded or shaped into any desired form. Among natural 

 plastics may be mentioned resins, gums, and rubber. Synthetic 

 rubber equal to the natural product in durability and cheapness 

 has still to be prepared (see p. 165), but many other plastics are 

 now of considerable industrial importance. 



Certain cellulose plastics have already been described (p. 399). 

 Cellulose behaves chemically like an alcohol, and as such forms 

 esters with acids (see p. 349). When cotton is treated with acetic 

 acid (in the form of acetic anhydride (CH 3 .CO) 2 0, see p. 315) it 

 gives cellulose acetate. The viscous liquid dries to a tenacious film. 

 On account of its waterproof character, non-inflammability and 

 non-conductance of electricity, it is now used for coating the wings 

 of aeroplanes, for making moving picture films, and for insulating 

 electric wires. Artificial horsehair (e.g. for making women's hats) 

 and bristles for hair brushes are made of it. 



By the action of nitric acid upon cellulose, various cellulose 

 nitrates may be formed, according to the number of OH groups 

 replaced by N0 3 (see guncotton, below). An incompletely 

 nitrated ester, when worked between rollers with camphor* and a 

 little alcohol, forms a viscous solution. When the alcohol evap- 

 orates, a transparent colorless solid, celluloid, remains. Photo- 

 graphic films are made by rolling the dough into sheets. Fillers 

 and dyes can be added to the dough and the latter can be moulded 

 to any form. In this way ivory-like or black combs and brush 

 handles, opaque white knife handles, articles of " artificial amber " 

 and so forth can be made. 



* A white solid with the formula doHi 6 O, obtained commercially by distill- 

 ing with steam the wood of the camphor tree, but recently also prepared syn- 

 thetically. 



