Observations on various Liquids. 231 



sion of glass, and weakened always to extinction by horizontal 

 tension. 



36. Oil of Colza. — As obtained from the merchant, this oil 

 also contained a large amount of solid sediment. After several 

 nitrations it was beautifully clear, and of a pale amber colour. 

 Tried in the plate cell, it acted as a very good insulator, the 

 sparks from the prime conductor being very little shortened 

 when the connecting wires were put in position. In electro- 

 optic experiment this oil was one of the cleanest that I had yet 

 examined which accounts partly for the fine effects observed. 

 These were of the same kind (contrary to CS 2 ) as in the pre- 

 ceding oils, but a great deal stronger. For the first time in 

 any fixed oil, I now saw the extinction-bands well developed 

 by the hand compensator. They were almost as fine as those 

 formerly observed in cumol; and, as was to be expected, they 

 afforded a good illustration of the difference between the fixed 

 oils and the former liquids. It may be remembered that in 

 cumol, as in carbon disulphide, the bands are developed and 

 moved in towards the axis of the field, against electric force, 

 by vertical tension of glass, or by horizontal compression. In 

 colza, on the contrary, the bands are developed and moved in 

 towards the axis by horizontal tension; and the action is pure 

 and constant here as in cumol. 



37. Oil of Mustard-seed, obtained as " Genuine East India" 

 from the apothecaries — transparent, and of a rich yellow colour 

 inclining to orange. Tested in the usual way, it acts as a 

 pretty good insulator, the spark-length of the prime conductor 

 being reduced not so much as one half when the connecting 

 wires are placed. In electro-optic experiments with this oil I 

 never got a perfectly clean charge ; but good optical effects 

 were easily obtained above and below the centre of the field. 

 The effects were pure and perfectly regular, and of the same 

 kind (contrary to CS 2 ) as in olive-oil, but apparently fainter. 



38. Raw Linseed-oil, of a brownish-amber colour, fairly 

 transparent, and only a moderately good insulator, the spark- 

 length of the prime conductor being reduced by it about three 

 fourths. This liquid was extremely troublesome in electro- 

 optic experiment, chiefly from the difficulty of cleaning it. 

 Not until many successive charges had been filtered and passed 

 through the cell did the polariscope give any sure sign ; and 

 at the best the effects were obtained only well above or below 

 the centre of the field. At last, however, the optical effect of 

 electric force on this oil was found to be quite certain and 

 regular, though extremely faint ; and it was certainly of the 

 same kind (contrary to CS 2 ) as in the other oils. A sample 

 of refined linseed also was examined, and with similar results, 



R2 



