PARAGUTTA, A NEW INSULATING MATERIAL 145 



TABLE II 

 Brittle Temper.vture of Paragutta and Other Insulating AIaterlvls 



Brittle Temperature 

 Material ° C 



Gutta Percha (Cable Insulation) —23 to —36 



Paragutta -45 to —61 



Balata (Washed) -44 to -52 



Balata (Washed and Deresinated) —62 to —67 



Crude Rubber -57 to -58 



Vulcanized Rubber (Soft) -53 to -58 



in the rubber or gutta percha and the salt concentration of the water 

 in which the samples are immersed are controlling factors. The 

 enormous increase in the quantity of water absorbed by ordinary 

 rubber when immersed in distilled water as compared with its absorp- 

 tion in salt solutions has been explained on the basis of osmotic 

 theory.^ In accordance with this theory rubber acts as a semi- 

 permeable membrane. Water soluble crystalloids or hydrophillic 

 colloids (proteins) attract the water which enters the rubber by 

 diffusion. When immersed in distilled water these impurities tend 

 to reach infinite dilution with water, being opposed in this by the 

 resistance of the rubber itself to swelling. In salt solutions the 

 amount of water absorbed is finite and depends on the equalization 

 of osmotic pressures of the internal and external solutions. The 

 change in water absorption of pure rubber hydrocarbon with the salt 

 concentration of the external solution is small over the whole range, 

 which indicates that the water enters by a process of solution. This 

 has also been found to be the case for gutta hydrocarbon and is more 

 or less true for paragutta and gutta percha. The water absorption 

 in distilled water can therefore be taken as a measure of the freedom 

 from water soluble or water absorbing impurities. Figure 8 shows 

 the effect of NaCl concentration in the immersion solution on the 

 quantity of water absorbed by samples of rubber, paragutta and gutta 

 percha at room temperature. Samples of rubber containing water 

 soluble matter or proteins do not readily reach an equilibrium water 

 content in distilled water. Crude rubber has been found to absorb 

 more than 100 per cent water in distilled water at ordinary temperature 

 without reaching equilibrium. ^ Gutta percha, paragutta and pure 

 rubber hydrocarbon on the other hand reach a definite and lower 

 equilibrium water content in distilled water, which shows their greater 

 freedom from water soluble or water absorbing matter. 



As the electrical stability of paragutta in sea water is of paramount 

 importance an exhaustive study has been made on a large number of 

 specimens as regards their changes in electrical values over long periods 

 of immersion in 3.5 per cent salt solution. Gutta percha insulation 



