April, 1910.] The Film Test for Crude Rubber. 147 



in transferring from container to slide. The term "figure" is 

 rather indefinite. In the experiment quoted it is not made clear 

 whether shape or color or character of film is to determine. 

 We assumed that a combination of both character of film and 

 its color was to be used. Shape of film is mere matter of 

 accident. Draft of air, declivity of table, skill of the operator, 

 all help to modify shape. 



"The viscosity of the solution will determine the thickness of 

 the film and will control the size, shape and number of vesicles. 

 The most important factors to be reckoned with are colour and 

 dirt (suspended matter)." 



We made the test very thorough, and examined in all, about 

 twenty-five kinds of crude rubber. Each test was made in 

 duplicate. The dry film was held for a moment in the fumes of 

 sulphur chlorid. This treatment did not alter the film but 

 removed the tackiness. Slides prepared in this wav keep 

 indefinitely, do not stick together and are free from dust. In 

 this way definite comparisons between a large number of films 

 could be easily and quickly made. 



We found that it was not a diflicult matter to obtain similar 

 duplicates from the same solution when made at same time. 

 Exactness was an impossibility. In some cases the difference 

 between duplicates were great enough to assign different names 

 to the same samples. We used much care in getting authentic 

 samples and in each case have compared the crude phvsical 

 characters with the descriptions given by reliable authorities;, 

 Pearson, Brandt, Falconette, and Clouth. 



Throughout the entire work we could not definitelv determine 

 a film peculiar to any one brand of crude rubber. Considerable 

 stress was put upon the " Lapori film." We found this figure 

 to be common to many kinds of rubber. Rubbers of different 

 botanical and geographical origin often gave this same tvpe of 

 film. 



Throughout the experiment there was an indication that the 

 character of the film was determined by the viscosity of the 

 solution. To test out this idea we made up a series of solutions 

 of these rubbers, varying from thin to verv thick. 



The table proves the point in question. It clearly shows the 

 influence of viscositv: 



