Oct. 1906.] 



297 



Saps and Exudations, 



and whatever other ingredients may be required. The rubber is then forced by 

 hydraulic pressure into huge iron moulds which will contain sometimes as much 

 as a ton of rubber, and which are rectangular or cylindrical according to the type 

 of machine which is to cut sheet from them. Special care has to be taken to prevent 

 the inclusion of air bubbles in this block of prepared rubber. When rubber is 

 compressed in this way into cylindrical moulds for manufacture of cut sheet an 

 axle of steel is forced through the centre of the mass while still in the iron mould: 

 The moulded mass has then to be annealed by gentle heating and maintaining 

 it at a moderate temperature for some little while. The next process is to harden 

 the block by freezing for a week in a refrigerating room, where it remains after 

 removal of the mould until wanted for use. The cutting edge of the knife, and 

 the surface of the rubber, are plentifully lubricated with water during cutting, 

 this also fulfils the further purpose of thawing the immediate surface of the rubber 

 and bringing the rubber to a suitable condition of hardness for the operation. The 

 sheets when cut must be carefully handled, being still soft and self-adherent, 

 unvulcanised, though perhaps containing mixed sulphur. 



DIPPED GOODS. 



33. There is still another mode of manufacture of hollow rubber goods 

 which may be called the dipping process ; it is simple in principle and very similar 

 to the way in which the old-fashioned tallow dip candles were made. A thick rubber 

 solution is prepared, usually of pure rubber and solvent, though pigments may be 

 mixed with it. A mould representing the internal shape of the required article 

 is dipped into this liquid and withdrawn. The solvent evaporates leaving a film 

 of rubber on the moulds, the operation is repeated until the required thickness 

 of rubber is accumulated. Any manipulation or cleaning of the edges is now 

 carried out and the rubber still on the mould is vulcanised. Here the "cold cure" 

 has to be adopted, since the rubber contains no sulphur already mixed, and the 

 customary solution of 3 per cent, of sulphur chloride in carbon bisulphide is em- 

 ployed, as previously desci'ibed under the head of vulcanisation. Certain classes 

 of surgical rubber goods are made in this fashion, and india-rubber teats for feeding 

 bottles are turned out by the thousand. A final dressing of rubber enamel is often 

 given to goods prepared in other ways, such as enemas and india-rubber balls, by 

 painting with or dipping in a rubber solution heavily loaded with pigments. 



ELECTRICAL USE. 



34. Rubber as an insulator of wires for cable use is being rapidly discon- 

 tinued, owing primarily to the high price of raw rubber. For sea cables rubber has 

 never been much used, gutta percha of course being superior, but land cable carry- 

 ing telephone wires, and which at one time were insulated with rubber are now 

 being largely insulated with dry paper. Heavy cables for electric light supply are 

 demanding for use in their manufacture less and less rubber every year, its place 

 being taken by papier-mache and cellulose pulp. For the flexible wiring containing 

 a single or a few strands of wire, such as are used in houses for electric bells, lights 

 and telephone communication, rubber is still employed, paper here is inadmissable 

 because it is less flexible, and also when exposed to the air becomes damp and an 

 inefficient insulator. The wire is coated with raw unvulcanised rubber by wrapping 

 a narrow strip, cut from thin sheet round the wire and pressing the adhesive edges 

 together. This is done by a machine which feeds the rubber slip from a spool on to 

 a travelling wire, the pressing together of the edge is done by running the wire 

 coated with the strip through guides and between wheels. Paper when used as 

 an insulator is wound round the wire spirally. The use of rubber for electrical 

 purposes in the form of ebonite fittings is considerable, but a great extension of the 

 electrical application of rubber consequent on any reduction in the price of the 

 raw material must not be expected. 



