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KKroitr OK CHEMICAL I-AIIOUATOUV 



ln88 tliiiii tlir true figurt'. It luiiy liiippcm that a .'50 \»r cent. Bolutioii, rcu'l in this way, may 

 show littlu more or even less viscosity tliiiii unv u( :i(J jkt criit., tin actual viscosity of which 

 is vi-ry nnich p-eiitcr. 



Obviously in order that a correct observation may he maile, the first reading should 

 always be at the same point. In the stronger solutions the number of degrees of retardation 

 is so great that the error introduced by starting from zero, nniy be ignored. The reailing will 

 then be the retardation noticed at the end of one conipK-te swing and the return to the same 

 point, thus : — 



First rending 



Second „ (K) 



Third „ 



Hvtardatiou ... 



And again First reading 



Second 

 Third 



(L) 



Ilctardation ... 



Average retardation ... 



Fur dilute solutions, it has been fi)unii more satisfactory to allow the disc to swing back 

 and forth a number — say 5 times, and note the total retanlation. In the case of 10 per cent, 

 gum solutions the total retardation divided by 4(38 has been found to give a much more 

 reliable figure than that derived from an observation on a single swing. 



In order to overcome the variations in different instruments, each one is standardized 

 against pure cane-sugar solutions, and the viscosity is expressed in the number of grams of 

 pure cane-sugar contained in 100 c.c. of the syrup at 60'^ F., which will give the retardation 

 designated at 80 ' F. These readings are obtained by making a number of solutions 

 containing knowni amounts of pure cane-sugar, and determining the retardation of each. A 

 curve is then marked out on a piece of plotting-paper, the number of grams of sugar in 

 100 c.c. of the different syrups representing the abscissas, and the degrees of retardation the 

 ordinates. This curve enables us to interpolate the value of each degree of retanlation in 

 terms of pure cane-sugar, and in this way a table of viscosities is drawn up and liirni.shed 

 with each instrument. This table renders the results obtained by the diflfereut instruments 

 strictly comparable. 



The above method serves to standardize the instrument, but as the curve of viscosity in 



the case of sugar is markedly different from that of gum, a second curve has been plotted 



Viscosity and which enables one to express the viscosities recorded in terms of average gum arable of good 



compared quality (sec chart). These figures give a much juster idea of the comparative strength of 



gums than the figure representing their viscosity. Thus, comparing crude Senegal gum 



" Gomme du bas du fleuve" with the best grade picked from it, we have the following 



figures : — 



Crude gum, TiM;osity in degrees of ret^irdatiou ... 31 

 Picked white gum ,. „ -, - ^6 



From which it would appear that the second gum was twice as strong as the lirsf. 

 Ai-tuallv, as niav be ascertained from the chart, the aniount nf gum n<|uircd to produce 

 the above viscosities were 



Crude j,'mii S-l 



Picked 100 



