FISHERY PRODUCTS IN EVERYDAY LIFE 31 



Fish-glue, the true liquid glue, has uses nearly as 

 extensive. It is also used in the manufacture of shoes 

 and slippers and in the preparation of paints. It is 

 not, however, a component of oil paints, but is used 

 in the preparation of cold water paints or calcimines. 

 Other important uses are the preparation of flexible 

 glues for book-binding, the manufacture of court- 

 plaster, the chipping of glass, box making, sizing, 

 and general repair work. It is the only ready-to-use 

 glue that is suitable for the many purposes for 

 which a quick-drying strong adhesive is needed. An- 

 other important use of fish-glue that is unknown to 

 most persons is in the preparation of enamels on half- 

 tone and zinc-line plates for use in photo-engraving. 

 Some potassium dichromate is dissolved in a fish- 

 skin glue solution. This solution is spread uniformly 

 on an absolutely clean copper plate, which is then 

 dried in a dark room over a low flame. Such a plate 

 is sensitive to the light, since the glue becomes insol- 

 uble in water upon exposure to light; it is exposed 

 behind a specially prepared plate in much the same 

 way as an ordinary silver chloride plate such as is 

 used for the making of lantern-slides. Another curi- 

 ous use of this glue is in the manufacture of belt 

 cement for joining leather belts, for which use an 

 especially strong flexible adhesive is required. 



Common salt is the most important inorganic 

 product obtained by the evaporation of sea-water, 

 some being prepared in every warm, dry country 

 bordering the sea. The American sea-salt industry 

 is centered near San Francisco and San Diego, Cali- 



