58 



subsequently in a desiccator, where it was preserved during the course 

 of the experiments. By these means we imitated as far as possible 

 the methods of purification employed by manufacturers. 



The masticating and mixing operations were carried out in a 

 specially equipped machine, which was found well suited for treating 

 small quantities for experimental purposes, and no difficulty was 

 experienced in obtaining a thoroughly even and homogeneous mass. 

 The machine was arranged so that it could be steam-heated and kept 

 under proper control. As some of the experimental results obtained 

 by Continental workers are open to criticism owing to the small 

 amount of material used, in our experiments not less than 50 grms. 

 of material were operated upon at a time to minimise any error due to 

 loss of sulphur or loading to be incorporated with the rubber. Some 

 Continental authorities have taken 5 grms. of rubber per mixing. 

 Under such circumstances a loss of finely powdered sulphur or 

 mineral is sure to take place ; when the quantities operated on are 

 very small the percentage loss may be considerable. Thus, 5 grms. 

 of rubber would require, say, only 0*3 grm. of sulphur, and a loss of 

 0*05 grm. would reduce the percentage of sulphur incorporated with 

 the rubber from 6 to 5 per cent. ; such a difference would entirely 

 vitiate the results. As the amount of mastication very considerably 

 influences the nature of the product in comparative tests, the mastica- 

 tion and mixing was continued for the same length of time. It has 

 been shown that prolonged mastication will increase the acetone 

 soluble products (so-called resins) by as much as 1 per cent. This 

 matter, however, requires further investigation. In most cases when 

 working with plantation rubbers without the addition of mineral 

 matter, the treatment during mastication and mixing seems to have 

 more effect on the product than the time of vulcanisation, provided 

 that it is kept within the usual safe limits ; that is to say, it would 

 appear that changing from two to three and four hours' vulcanisation 

 had a smaller influence on the tensile strength of the product than a 

 difference in treatment in mastication and mixing. These points 

 require more experimental work, which is in progress, but will not be 

 discussed in this paper. We shall merely deal here with the general 

 results. 



The mixture, after kneading and while still hot, was forced into 

 an iron cylinder. The bottom of the cylinder was closed with a 

 screw-cap, and by means of a steel plunger the mixing was compressed 

 into a solid cylindrical block of such a size that slices could be cut off 

 to fit into the vulcanising moulds. The moulds are of similar con- 

 struction to those used on a large scale. 



On a manufacturing scale it is usual to give the digester a gradual 

 " rise " ; that is to say, the temperature is never suddenly raised, but 

 is taken up to one corresponding to a certain pressure, say, for one 

 hour before raising it further. We have tried both slow and rapid 

 methods with mixings containing little or no mineral, and have not 

 found the gradual raising of the temperature to influence the strength 



