Gums, Resins, 



282 



[Oct. 1906. 



increasing rapidity, squeezing out from the interior of the mass the bulk of the water 

 and soluble constituents of the latex, until it forms a fairly compact white mass 

 floating on the surface of the clear water. This, when washed, pressed and dried, 

 constitutes rubber and contains 95 to 96 per cent of pure caoutchouc with a specific 

 gravity of '92 to '96. Raw rubber so prepared has a characteristic odour, but when 

 produced by the fermentation process and imperfectly washed is highly offensive. 

 It is insoluble in water, but its bulk increases by absorption when immersed in that 

 fluid, and it adds 25 per cent to its weight, while its toughness, adhesiveness and 

 elasticity are greatly reduced. Raw caoutchouc is very elastic at normal tempera- 

 tures ; but if the temperature is reduced to zero, it becomes hard and brittle, but 

 regains its elastic property on being warmed. If heated to 80° Centigrade, or more, 

 it becomes soft and sticky and does not regain its normal properties on being cooled. 

 Exposed for a long time to warm air and light, it becomes less elastic on the surface 

 and more or less sticky or tacky. This condition may also be caused by bacteria or 

 fungi, with probably the production of an oxidising enzyme. 



Spiller states that the affected caoutchouc consists of : — 



Affected. Unaltered Caoutchouc. 



Carbon 64'0 ... ... ... 85'5 



Hydrogen 8'5 ... ... ... 12*0 



Oxygen 27*5 ... ... ... T5 



Heated to 360°-Fahr. caoutchouc begins to melt, and between 400 and 400°-Fahr. it 

 becomes a dark brown oil. Owing to the above properties of rubber at different 

 temperatures, crude caoutchouc would be of little general use had not the curious 

 reaction of caoutchouc and sulphur known as " vulcanisation " been discovered. 



The only uses for raw rubber are the cubes employed by artists, strips for 

 billiard cushions and square cut unvulcanized threads. 



The treatment of rubber with either Sulphur or Sulphur Chloride are the only methods 

 at present employed for vulcanisation, though Gerard's process of treating the India- 

 rubber for three hours in a solution of calcium pentasulphide (1'205 sp. gr.) under 

 a pressure of 60 lb. (140° C) yields very satisfactory results. The chief vulcanisers 

 and sulphur-carriers are sulphur, which occurs in two crystalline and one amorphous 

 forms and various metallic sulphides. The chiefly-used commercial varieties are 

 sulphur sticks, flowers of sulphur, milk of sulphur and precipitated sulphur, the 

 latter being undoubtedly the best form of sulphur for vulcanising purposes, It is 

 prepared by the precipitation of sulphur from the solutions of polysulphides with 

 acid, when it separates in an exceedingly fine state of division, and so is capable of 

 more intimate mixture with the Indiarubber. Lead thiosulphate (Pb S2 O3 ) and 

 oxide, antimony pentasulphide, (Sb2 So ) and zinc sulphide are among the chief 

 sulphur carriers. All the above generally require what is known as hot vulcanisa- 

 tion, Chloride of sulphur (S2 CI 2. ) on the other hand is extensively used for the 

 process of vulcanisation in the cold, but at present can only be employed for vul- 

 canising dry goods, as contact with water causes immediate decomposition. Besides 

 the ordinary sulphuring agents, various colouring matters are frequently 

 incorporated, when the rubber is to be used for decorative or ornamental purposes ; 

 but for technical purposes the colour is more or less confined to grey, black 

 and red. 



VULCANISING COLOURING INGREDIENTS. 



Examples of sueh colouring ingredients are zinc white, lithopone (Zns. Ba SOi ) 

 and green pigments, golden sulphide (Sb2 S5 ), vermilion, red and brown iron oxides, 

 and various yellows and blues. 



Rubber may be looked upon as a colloid body, a class which have a high 

 molecular weight and are non-electrolytic. In colloids only one molecule out of 



