SCIENCE IN THE INDUSTRIAL WORLD 



varnish-makers to determine the stage of melting by 

 the use of thermometers (as is done in experiments 

 on a small scale in the laboratories) but to depend 

 upon the "feeling" by means of the stirring-rod in 

 the hands of an experienced workman. 



When the resin is dissolved, the kettle is withdrawn 

 from the fire and allowed to cool a little, and the foam 

 to settle. Meanwhile the requisite amount of linseed 

 oil is being prepared in another kettle. This prepara- 

 tion consists in heating it to a temperature varying be- 

 tween 100 F. and 500 F. according to the quantity, 

 quality of varnish to be made, and individual preference 

 of the manufacturer. The oil is added to the melted 

 resin, the mixture being stirred constantly during the 

 process. To all appearances a perfect solution results; 

 but this is only apparent. If the mass were allowed 

 to cool at this stage there would be a separation of the 

 resin from the oil, and a cloudy mixture would result. 

 Therefore the whole mass must be placed over the fire 

 again, and heated until a drop placed upon a piece of 

 glass no longer shows a cloudy appearance on cooling. 

 In practice the manufacturer no longer depends upon 

 this test, but is guided by the reading of a thermometer, 

 keeping the mixture at a certain temperature for a period 

 of time that experience has taught him is right for mak- 

 ing the particular quality of varnish in hand. 



The immediate effect of the cooking, besides the 

 essential one of causing the two substances to form a 

 solution, is to thicken the liquid to make it so viscid 

 that it will require some thinner liquid, such as turpen- 

 tine, to make it ready for commercial purposes. Both 



