BAIRD &TATLOCK (LONDON) LTD. 



a movable vertical shutter, so placed as to divide the luminous field of the instrument into two parts, the one 

 being quite bright, while the other is in shadow. It is the position of the edge of this shadow on the scale that 

 determines the readings with the apparatus. If the same liquid be placed in both reservoirs the shadow will come 

 on the zero of the scale ; but if a different liquid be placed in the inner prismatic reservoir, the shadow will come 

 at some point either to the right or to the left of the zero, according to the nature of the refractive power of the 

 liquid. The appearance of the shadow in the field is shown in the drawing. The illumination is obtained by 

 an ordinary gas jet placed opposite to the collimator end of the instrument. Both receptacles are furnished with 

 draw-off cocks, and are enclosed in an outer case (not shown in the drawing) , into which water is put for the purpose 

 of regulating the temperature, and which case is heated by a little lamp placed underneath, so as to maintain 

 its contents at any desired degree. M. Jean has invented a typical oil that gives no refraction, and with which the 

 outer receptacle is always filled, and the temperature is brought to 22 C. If, now, some more of this oil be also 

 brought to 22 C., and placed in the prismatic reservoir, the shadow will mark zero, or if it does not do so it is 

 adjusted to that point by moving the shutter. The typical oil is then run out of the prism, and the oil to be 

 tested, having been brought to 22C., is placed therein, and the reading is taken. All the vegetable oils, and some 

 marine animal oils, deviate the shadow to the right of the zero, while the terrestrial animal oils act in the contrary 

 sense. It takes about half an hour to regulate the instrument and get all the temperatures equal in the reservoirs ; 

 but once this is attained, one sample after another can be done at intervals, of five minutes, a portion of every 

 sample of oil being used to rinse out the inner prismatic reservoir before actually filling it for observation. The 

 following table will give some idea of the differences that can be got between the various vegetable oils : 

 Olive .. .. .. .. .. .. .. .. + 1.5 to+2 



Colza .. .. .. .. .. .. .. .. + 16.5 to + 17.5 



Ground nut . . . . . . . . . . . . . . +4-5 



Sesame .. .. .. .. .. .. .. .. +17 



Cotton ...... ... +20 



Castor 

 Linseed. . 

 Hempseed 

 Poppy .. 

 Almond 

 Japonica 



Of the terrestrial animal oils and fats which go to the left we have 

 Neats-foot 

 Horse-foot 

 Lard 



Beef tallow 

 Mutton tallow 

 Butter fat 

 Margarine (average) 

 Oleic acid 



+40 



+53 



+33 



+30 to + 34 



+ 6 



+50 



12 



-'2-5 

 16 



20 

 - 35 

 -15 

 -34 



Before observing any oil, it is to be, if at all rancid, shaken up in a separator with hot alcohol, to remove the 

 free fatty acids, and dried at noC. This is especially necessary in the examination of the lower qualities of 

 olive oil, and indeed, with this particular oil, it is always desirable to proceed in this manner, if a preliminary 

 examination of the oil itself does not indicate the proper refraction. 



4991' 



4991 



Ackermann's Calculating Disc, for the rapid determination of alcohol and extract 

 contained in beer, by Zeiss. (Description on page 902) 



3 10 



