280 THE PHYSICAL EXAMINATION OF BUTTER FAT. 



The connection between x and [n] D is expressed by the formula, 

 287-7 x = 8394 Vl-5395 -- \n\^\ this can be easily worked 

 out with a table of 4-figure logarithms. This formula has been 

 simplified by the author from those of Koberts and Liverseege. 



There is a difference in the refractive index depending on the 

 light used ; this is corrected in the instrument by making the 

 prisms of different kinds of glass, so that when used with butter 

 ordinary white light behaves as if it were simple light. Other 

 fats (and adulterated butters) may be tinged at the edge with 

 blue or red. In this case it is not easy to read the dividing line 

 accurately. The author is in the habit of using the sodium 

 flame, obtained by heating sodium chloride in a Bunsen burner, 

 as the source of light, and finds that absolutely sharp readings 

 can thus always be obtained. The readings with butters do not 

 differ, whether white light or sodium light be used. 



The refractive index varies 0*55 scale degree for each 1 C., 

 and can be corrected by means of this factor if the temperature 

 differs from that adopted as normal. 



For the correction of scale readings taken at a temperature 

 to any other temperature or to that adopted as a standard, Leach 

 and Lythgoe have devised a slide rule, but, except for small 

 differences of temperature, the author finds that the readings 

 are not strictly correct. 



A chart for the correction of butyro-refractometer readings 

 for temperature may be constructed thus : Select a sheet of 

 squared paper at least 120 units by 200 units wide ; at a point 

 34 units from the bottom, set out horizontally a series of points 

 5 units apart, and at a point 119 units from the bottom a similar 

 series of points 7 units apart ; join the corresponding pairs of 

 points to form a series of temperature lines. The middle vertical 

 line is selected as the standard temperature, say 35, and each 

 line to the right will represent a temperature of 1 higher, and 

 to the left 1 lower than the next preceding line. 



From the bottom at a point 100 units from the standard 

 temperature line to the left draw a line to the point which lies 

 20 units from the bottom and 100 units to the right of the standard 

 temperature line ; this will represent on the scale of the 

 butyrometer ; draw parallel to this a series of lines 10 units 

 apart measured vertically, and mark these 10, 20, etc., of the 

 refractometer scale. To use the chart, find the point of inter- 

 section of the lines corresponding to the observed temperature, 

 and scale lines (differences between the lines can be estimated 

 with sufficient accuracy by the eye), and the distance measured 

 horizontally between this point and the vertical standard line will 

 give the correction to be added if on the right, or subtracted if on 

 the left ; 10 units of distance equal one scale degree of correction. 



