CANE SUGAR. 



datum can be obtained in the laboratory several hours after the strike has been 

 finished, and in place of a laboratory analysis there is substituted the sugar 

 boiler's sense of touch. In 1898 Curin 4 introduced into the beet sugar industry 

 an instrument called the brasmoscope designed to control the operation of pan 

 boiling and founded on the annexed principles. 



Soiling point. All liquids are constantly giving off vapour from their 

 surface and when the pressure of the vapour equals that of the surrounding 

 atmosphere the liquid boils ; as the pressure of the surrounding atmosphere 

 increases so does the boiling point of the liquid, and conversely with a fall of 

 pressure there is a corresponding fall in the boiling point ; when water or 

 other liquid boils under a pressure less than that normally due to the atmos- 

 phere, it is said to boil under reduced pressure or less correctly ' in vacuo.' It 

 is customary to express the pressure under which sugar solutions are boiled in 

 ' inches of vacuum.' The normal pressure of the atmosphere will support a 

 column of mercury 29-92 inches high ; an absolute vacuum would then be 

 expressed as 29'92 inches, and a vacuum of 25 inches will mean that the 

 excess pressure of the atmosphere over the pressure in the vessel, in which 

 there is a vacuum of 25 inches, is 29'92 25 = 4'92 inches. This method of 

 expressing pressure less than one atmosphere is not altogether convenient, and 

 for many reasons it would be better to speak of a pan being boiled under a 

 pressure of 5 inches absolute, rather than as under a vacuum of 25 inches. 

 In the Appendix is given a table connecting the pressures and temperatures at 

 which water boils. 



^Effect of dissolved solids on the boiling point. The boiling point is increased 

 by the presence of dissolved solids and the following important relation con- 

 nects boiling point, amount of dissolved solid and pressure under which 

 ebullition occurs. ' The elevation of the boiling point due to the dissolved 

 solids is independent of the pressure under which ebullition occurs.' For 

 example, under a pressure of one atmosphere water boils at 212 F. and a 

 75 per cent, solution of sugar at 225'2 F. The elevation in the boiling 

 point is then 13-2 F. ; under a pressure of 4 inches of mercury (25'9 

 inches of vacuum) water boils at 125-6 F. ; a 75 per cent, solution of 

 sugar under the same pressure will then boil at 125-6+ 13-2 = 138'8 F. 

 The temperatures at which sugar solutions of different concentrations boil under 

 atmospheric pressure have been determined (see Table in Appendix) ; if then 

 the temperature of a boiling sugar solution be known, and also the pressure 

 under which ebullition occurs, then from the elevation of the boiling point 

 over and above the boiling point of water under the same pressure, the 

 amount of sugar in the boiling mass can be at once found. For example, 

 under a pressure of 4 inches of mercury a sugar solution boils at 159'4 

 F. ; water under this pressure boils at 125'6" F. ; the elevation in the 

 boiling point then is 29-8 F. ; reference to the table of elevation of boiling 

 points of sugar solutions gives the percentage of sugar as 8 6 '2 5 per cent. 



360 



