88 VISCOSITY OF RUBBER SOLUTIONS 



separating it as the tetrabromide or as nitrosite, and 

 methods for these purposes have been devised by a number 

 of chemists. It is, however, difficult to obtain concordant 

 results with these methods, and they have in consequence 

 been generally abandoned so far as the examination of 

 raw rubber is concerned. 



Anotlier drawback to the present method of analysing 

 crude rubber is that rubbers of very different quality often 

 give similar figures on analysis. For example, it has been 

 already pointed out that rubber from very young Para 

 trees in Ceylon is apparently identical in composition 

 with that from the older trees, the percentages of resin 

 and caoutchouc being practically the same in each case. 

 The rubber from the young trees is, however, usually 

 much weaker than that from the older trees, and it would 

 seem that this difference in strength must be due to some 

 difference in the caoutchouc, which is not revealed by the 

 present methods of analysis. Until some progress can 

 be made in this direction it will not be possible to corre- 

 late the results of the chemical analysis with the physical 

 properties of the rubber. 



Viscosity of Rubber Solutions. — It is weU kno\^^l that as 

 a general rule the best qualities of rubber fm^nish the 

 most viscous solutions, and it was suggested hj Axelrod * 

 and subsequently by Schidrowitz and Goldsbrough f 

 that the viscosities of the solutions might be used to 

 measure the strength or " nerve " of the crude rubbers. 

 For the purpose of the determinations Schidrowitz and 

 Goldsbrough recommend dilute solutions of rubber (025 

 to r5 per cent.) in thiophene-free benzene, and they deter- 

 mine the viscosities in an Ostwald viscosimeter at a 

 temperature of 20° C. The comparative viscosities are 

 measured by the rate of flow of the solutions compared 

 with that of benzene in the same apparatus. At least 

 three determinations are made with solutions containing 

 approximately 025, Oo, and TO per cent, of rubber, and 

 the results arc plotted as a curve. Schidrowitz has proposed 

 to express the viscosity bj'^ a number obtained by di'awing 

 a tangent to the curve at a concentration of 1 per cent, 

 and taking the value of this tangent on a standard scale. 



* Gummi Zcitung, 1005, vol. xix. p. lOaS; vol. xx. p. 105. 

 t Journal of Society of Chemical Industry, 1909. p. 3 ; and Rubber, 

 by P. Schidrowitz (London: Mothueu & Co., Ld.). 



