176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



composed of five laminations. The center one is usually a piece of 

 plate glass about three-quarters of an inch in thickness. To each 

 side of this is cemented a sheet of cellulose plastic and on the outside 

 of each of these layers of plastic there is cemented a piece of thinner 

 plate glass, making a composite piece a little over an inch thick. 

 Such material resists bullets even at close range and is finding use 

 in the windows of armored cars and cashiers' cages in banks. Up 

 until very recently the plastic used for this purpose was made from 

 cellulose nitrate. This was not ideal for the purpose because it 

 gradually discolored in the light. However, considerations of cost 

 compelled its use. Within the last year or so, however, a revolu- 

 tionary change in the method of manufacture of these plastics has 

 so lowered the cost of the finished sheets of cellulose acetate plastic 

 that at the present time it is estimated that over 70 percent of all the 

 laminated glass manufactured in this country is made with cellulose 

 acetate plastic. The manufacturing improvement responsible for 

 this surprising change consists in replacing a long series of batch 

 operations by a single continuous process which cuts the time of 

 production from 10 or 12 days to the corresponding number of 

 hours ; and since the material is totally enclosed during its manufac- 

 ture, practically the total production is of first quality. 



When attention is turned to the manifold applications of cellulose 

 and its derivatives in modern civilization, it becomes quite apparent 

 that cellulose is not only nature's riddle but also nature's paradox. 

 Cellulose in the form of wood has long been used as a material of 

 construction because of its resistance to atmospheric conditions, yet 

 at temperatures slightly under that of boiling water, cellulose slowly 

 combines with oxygen and decomposes. Again cellulose in the form 

 of cotton is widely used for clothing because of its resistance to 

 washing and to the chemicals usually used in that operation, yet two 

 samples of cellulose, identical except that one has been boiled in 

 distilled water for 2 hours and then dried, exhibit markedly different 

 chemical characteristics. 



We therefore find that to supplement the age-long uses of cellulose 

 which have been dependent on its resistance to chemical change, 

 there is now springing up a long list of uses which depend upon its 

 chemical reactivity. If cellulose in the form of cotton thread or 

 fabric is treated with an 18 percent solution of caustic soda in the 

 cold and dried under tension, there is obtained the well-known effect 

 of mercerization which produces a silky finish on cotton fabrics. 

 On the other hand, if sulfuric acid of about 70 percent strength is 

 used instead of the caustic soda and the acid thoroughly washed out, 

 there is obtained the material known as vegetable parchment, which 

 is sufficiently water resistant so that it has been sold to replace cloths 



