218 THE WEALTH OF THE SEA 



very brittle and crack because of the expansion and 

 contraction caused by alternate heating and cooling. 

 Fish-oil films are more elastic and consequently do 

 not check so quickly. On this account they are used 

 in nearly all paints exposed to extreme changes of 

 temperature, such as those for roofs and smoke- 

 stacks. 



Liver Oils 



Cod-liver oil, like fish-oil, has had a bad name for 

 a long time. Its undesirable reputation is not entirely 

 deserved but is largely a heritage of the early days 

 of the industry when it was made by placing the 

 livers in barrels and allowing them to rot. Under 

 these conditions the liver cells burst, and the oil 

 which extruded was skimmed off. In winter, the first 

 oil obtained was light in color and was used as medi- 

 cinal oil. The darker oil obtained later was called co3 

 oil and was used for the curing of leather. When no 

 more oil exuded, the remainder was obtained by pla- 

 cing the residue in an iron kettle and heating it over 

 a fire. This dark rancid oil was also sold as cod oil 

 for industrial purposes. 



All this has been changed within the last few 

 years. A hundred or more chemists have been study- 

 ing methods of production, processes of removing 

 the color and odor, and methods of separating the 

 vitamines to which this oil owes its valuable therapeu- 

 tic properties. These researches have accomplished 

 wonders, resulting in novel processes of manufacture 

 which have greatly improved the quality of the oil, 

 so that now the best oil has relatively little odor and 

 taste. 



