83 



almost entirely into nitric esters. Leaving this complication out of 

 account, however, it is evident that the reaction must proceed by the 

 diffusion of the acid mixture through the fibre. The final degree of 

 nitration reached depends principally on the composition of the acid 

 mixture, and nitroceUuloses of all percentages of nitrogen from 

 10 to 13 occur in commerce. From what has been said it is evident 

 that the nitration process is very complicated, and it is not to be 

 wondered at that it has not been brought to mathematical expression, 

 although a vast amount of data has been accumulated*. However, 

 by the accurate control of the composition of the bath, the purity 

 and humidity of the cellulose, the temperature and time of nitration, 

 very uniform products are obtained, and, to take only one instance, 

 that of the manufacture of guncotton, the accuracy of modern 

 musketry and artillery practice proves how uniformly a two-phase 

 reaction can be controlled, although involving diffusion through a 

 membrance which itself alters during the reaction^. 



Acetyl- cellulose is prepared by the prolonged action of acetic 

 anhydride, acetic acid and sulphuric acid on cellulose, but the process 

 differs from nitration in that the product is soluble in the reaction 

 mixture. In this case, therefore, the acids reach the cellulose by 

 diffusion through a gel of acetyl cellulose in acetic acid. The final 

 product of the reaction is homogeneous, at any rate down to ultra- 

 microscopic limits, and the acetyl-ceUulose is recovered in the solid 

 state by precipitation with water. The acetylation of cellulose takes 

 much longer than nitration, and up to the present the uniformity of the 

 product is inferior to that of nitrocellulose. This is probably due 

 partly to the difficulty of temperature control, and partly to the 

 compHcations introduced by modifications designed to produce material 

 soluble in special solvents^. 



The central point in the manufacture of celluloid and acetyl- 

 ceUulose plastic materials is undoubtedly the gelatinisation of the 

 base. Nitrocellulose retains the form of the original cellulose, 

 although harsher to the touch. When it is kneaded with camphor 

 and alcohol it is converted into a transparent gel, and the remaining 

 processes of the manufacture merely consist in manipulating the 

 gel while it is slowly hardening through loss of alcohol and part of 

 the camphor. It is rolled out into sheets, pressed into blocks, sliced 

 on a planing machine, and finally poUshed if required. The treatment 

 of acetyl- cellulose is similar in principle, although different solvents are 

 employed. 



During the evaporation of the volatile solvents from celluloid in 

 its seasoning or drying stage, there is a gradual loss in weight, diminu- 

 tion in volume, and increase in specific gravity. It may be noted in 

 passing that the specific gravity of celluloid is of particular interest 

 to the manufacturer of celluloid articles, since he buys the material by 

 weight, and, in effect, sells it by volume. The relation of the loss of 

 weight to the loss of volume was investigated by the writer in relation 

 to another technical problem. The extreme cases would be that of : 



(1) a sponge-like structure which could lose weight without (apparent) 

 loss of volume in which case loss of weight/loss of volume = oo ; 



(2) a structure which could shrink in volume without loss of weight, 



F 2 



