262 ANIMAL PROTEINS 



processes, in which the principle is that gelatine is made 

 insoluble in water by the action of potassium dichromate 

 under the action of light. It is used also in Poiteoin process 

 for copying engineering drawings, which is based upon the 

 power of a ferric salt to render gelatine insoluble so long 

 as it is not exposed to the actinic rays. 



Gelatine is used in the manufacture of the " crystalline 

 glass " used for decorative purposes. Advantage is taken 

 of the immense contractile force it exerts on drying. When 

 ground glass is coated with gelatine, and the latter dried, it 

 tears away the surface of the glass itself, and leaves peculiar 

 fern-like patterns. Inorganic salts dissolved in the sol 

 influence the nature of the pattern obtained. 



Gelatine is used also very largely in the textile trades, 

 for finishing coloured yarns and threads, for sizing woollen 

 and worsted warps, and for thickening the dyestuffs used 

 in printing fabrics. It is also used for finishing white straw 

 hats ; as a size in the manufacture of high-class papers, 

 and as a wax substitute for covering corks and bottle necks. 



Glue is used instead of gelatine in all cases where colour 

 is not a matter of much moment. The fact that it has not 

 been bleached makes no difference to its suitability in such 

 a case, and the cost is substantially reduced. Thus, for 

 dark-coloured straw hats, textiles, sweets, papers, and in 

 all suitable woolwork, glue is used in place of the more 

 expensive article. 



A very large quantity of glue is used in the manufacture 

 of matches, where it functions as the material binding the 

 " head " to the stem. A 15-50 per cent, sol is used, con- 

 taining nitrate or chlorate of potash as oxidizing agent. 

 The mixture is kept at 38 C. and the phosphorus cautiously 

 added, and when this is emulsified, the friction ingredients 

 (sand, glass, etc.) are also added. The glue acts also in 

 preventing premature oxidation. Glue is also used in 

 making the match-boxes, and similarly in making sand, 

 emery, and glass papers and cloths. 



There is a large consumption of glue by joiners, carpenters, 

 cabinet-makers, and all kinds of woodwork and fancy work. 



