5 « 
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 
