Oct. ia06.] 



283 



Saps and Exudations. 



many is supposed to be capable of acting chemically : hence their great chemical 

 indifference. They have the property of gelatinizing or pectising ; but the latter is 

 quite distinct from the former in that it is non-reversible, while the gelatinized state 

 can be reversed by heating or other means. Colloid solutions can be pectised or 

 converted into insoluble colloids at much less concentration than is required for 

 gelatinization ; but the influences that induce pectisation are varied. Some colloidal 

 solutions pectise spontaneously on standing, some by boiling and many by freezing, 

 while most inorganic colloids pectise on adding to their solutions minute quantities 

 of electrolytes. The question of the change involved in the pectisation of a colloid 

 is still obscure, but it will probably be ultimately proved to be partly physical, but 

 mostly chemical— the changes in the molecules resulting in the fixation of the rela- 

 tive positions of the colloidal molecules. It is possible that the Brownian movement 

 of the rubber particles, increased by the change in reaction or the addition of an elec- 

 trolyte, may tend to induce coagulation by increasing the attraction of the various 

 molecules for one another, and by their friction produce amalgamation or cemen- 

 tation. Indiarubber is very sensitive to changes of temperature, becoming soft 

 and sticky or tacky at high temperatures, and hard and brittle at low temperatures, 

 i.e., 10° C or 50° Fahr. These properties render it quite unsuitable for most economic 

 uses both in hot and cold countries ; and had it not been for Goodyear's and 

 Hancock's discoveries regarding the effect of heating mixtures of rubber and 

 sulphur together to a temperature above the melting point of the latter, the 

 demand for rubber would have been very limited. This process is now commonly 

 known as vulcanisation, and consists in the formation of a continuous series of 

 addition products involving the chemical combination of these two substances— 

 polyprene and sulphur. When heated (under pressure or otherwise) to a temper- 

 ature of 120° C. or over (the melting point of sulphur being 113-5° 0.) the resulting 

 addition compound is a polyprene sulphide of the probable formula (Cioo Hieo S) 

 for soft goods and (Cio Hi6 S2) for ebonite, the highest vulcanised product. The 

 rate at which sulphui enters into combination with the Indiarubber hydrocarbon 

 "polyprene" ((J10 Eta) varies with each brand of rubber and the temperature and 

 time employed in the vulcanisation, but in most instances there is a steady 

 increase corresponding with the higher temperature and longer period employed, 

 and in the finished product a gradual reduction in elasticity with increase of 

 hardness. 



METHODS OF VULCANISING. 



In the ordinary vulcanisation there is only from 2 to 2'5% of combined 

 sulphur, but a much larger proportion is usually added in the mixing, and some of 

 this uncombined sulphur frequently appears on the surface of rubber goods 

 in the form of a fine efflorescence. There are three or four methods by 

 which this is accomplished. The first is to introduce the articles into 

 a boiler, and after closing the end hermetically to admit steam from another 

 boiler until the pressure amounts to 52 or 60 lb. per sq. inch, corresponding to a 

 temperature of 142° C. to 141° 0. The length of time this has to be maintained 

 depends on the quality of the rubber, Para vulcanising more slowly than the soft 

 and more sticky varieties ; it also depends on the cross-action of the rubber to be 

 vulcanised— thin objects being completed within the first hour, and thicker objects 

 requiring two to three hours. 



The boilers employed for this purpose are frequently of enormous size, the 

 largest being about 65 feet long by 16 feet to 20 feet in diameter. They are con- 

 structed of strong wrought iron in a similar manner to an ordinary steam boiler 

 except that one end is removable for putting in on a tramway the trays or trollies 

 on which the various articles are placed. The object of the great length is to 

 vulcanise tubes in a straight line, and so preserve their shape, the tube being 

 mounted on an iron mandrel, the diameter of which corresponds to the inner bore. 



