466 
appeared to have solved the old difficulties associated with drying. 
There had been, however, suggestions and proposals to introduce 
vacuum drying on estates, and consequently I paid special 
attention to the modes of drying washed rubber in use in Great 
Britain. In a few manufactories only I found vacuum drying 
had been introduced, but the great bulk of washed rubber is 
still dried by hanging it up in dark warm rooms. A vacuum 
drying chamber is a large iron box, of from ioo to 200 cubic 
feet capacity, fitted inside with shallow iron trays with per- 
forated bottoms, and heated with steam pipes. The interior is 
connected by an iron pipe with an exhaust pump. Wet rubber is 
placed in the iron trays, the doors are closed, and the temperature 
raised to 120 to 130 degrees F., and the pump started. The air 
and water vapour that are drawn out of the chamber are passed 
through a condensing • cylinder, fitted with a glass front, and the 
condensed vapour is seen falling as a stream of drops of water. 
After two or three hours this stream of drops of water ceases, and 
the rubber or whatever material is being operated upon is then 
regarded as dry and ready for removal. Rubber dried in this way 
is always softened by the heating which is necessary if the drying 
is to be rapid, and in the opinion of manufacturers who have not 
adopted the process the nerve and quality of the rubber are injured, 
though with those who have adopted vacuum drying this is not 
regarded as very important, chiefly because the cause of softening 
is known, and it is regarded as only an anticipation of the softening 
which always occurs in mastication of the rubber, the next step in 
the process of manufacture. On the other hand, some forms of 
rubber — such as very soft African rubbers — cannot be dried in this 
way at all because the softening in their case proceeds too far. 
The sheets of rubber dried in this way adhere to one another when 
packed and stored away, this is of no importance in the factory, 
because the rubber is known to be clean and dry and ready for use, 
but if plantation rubber were offered in the same soft and adhesive 
masses, objection, and serious objection, would naturally be made. 
It would be necessary to maintain a lower temperature in the 
vacuum chamber than is in common use if this softening of the 
rubber is to be entirely avoided, and this w r ould seriously interfere 
with the efficiency of the machine. The rate of the evaporation of 
the water and drying of the contents depends upon the difference 
between the temperature of the vacuum chamber and the tempera- 
ture of the condensing chamber, as well as upon the absolute 
temperatue of the vacuum chamber. In the tropics it will not be 
possible to maintain the condenser below 80 deg. F., which is 20 
degrees above the temperature of the condenser in England. This 
diminished difference, together with the necessary lowering of the 
temperature of the vacuum chamber itself, will seriously impair the 
efficiency of the vacuum drier, the only advantage of which is the 
rapidity of drying. Taking into consideration the further fact that 
plantation rubber is always inclined to be soft, I should certainly 
not recommend any form of drying in which artificial heat is 
necessary, and which involves the elaboration of machinery and 
