ii BUTTER 145 



alcohol, phenolphthalein being used as indicator. The number 

 of cubic centimetres of N/2Q potash is added to the critical 

 temperature which has been found, and the sum expresses the 

 real critical temperature. As it is very tedious and difficult to 

 determine the specific gravity of the alcohol before each test, 

 Crismer 1 has proposed to use a standard petroleum, by which 

 means first the specific gravity of commercial absolute alcohol 

 is determined, and afterwards the critical temperature of the 

 mixture of petroleum and alcohol. 



The method of constructing a curve which can be used for 

 obtaining the required correction for alcohol of any strength 

 can be seen in Crismer and Vandam's original paper. 2 



The critical temperature of solution of butter-fat in alcohol 

 lies generally between 53 and 57 C. ; in exceptional cases the 

 upper limit may reach 59 C. 



IV. DETERMINATION OF THE REFRACTIVE INDEX. 



The refractive index is generally determined with a Wollny 

 butter refractometer made by Zeiss, of Jena. With slight 

 modifications, this instrument is the same as the milk refracto- 

 meter described on p. 40. 



The graduation of the scale of the butter refractometer is 

 from 5 to + 1 05, corresponding to refractive indices of 

 1'4179-1'4922. It is therefore a continuation of the scale of 

 the milk refractometer. 



The method of using the butter refractometer is exactly the 

 same as described for the milk refractometer, a few drops of 

 clear filtered butter fat being placed between the prisms. A 

 refractometer and an apparatus for supplying water at a 

 constant temperature are all that is required, and the deter- 

 mination is generally made at 40 C. The coloured critical 

 line between the illuminated and the dark fields is rendered 

 colourless in the butter refractometer by a special arrangement 

 of the upper prism, so that with normal butter the critical 

 line appears sharp and colourless. 



To make a determination, the faces of the prisms must be 

 cleaned with the greatest care ; a soft, clean piece of fine linen, 



1 Bulletin de la Soci6t6 chimique de Belyique, Vol. XVIII, Jan., 1904. 



2 Ibid, Vol. XX, 1906 ; Third International Dairy Congress, Section 1, 

 No. 16. 



