COLOUR TESTS. 261 



residue is extracted several times with petroleum ether, which 

 is evaporated, and the test performed on the residue. They 

 claim that 0<1 per cent, of sesame oil can thus be detected. 



Arnold extracts the fat (dissolved in petroleum spirit) with 

 hydrochloric acid containing Ol per cent, of stannous chloride, 

 which destroys the red colour on heating, after which the furfural 

 is added. 



Becchi's Test for Cottonseed Oil. This test was originally 

 performed by heating the fat with a solution containing silver 

 nitrate, alcohol, ether, nitric acid, amyl alcohol, and rape oil. 

 The reagent has been frequently modified. Bevan prepares the 

 reagent by boiling silver nitrate with amyl alcohol, and cooling 

 the solution. Equal parts of this solution and of the fat are 

 heated in a test-tube on a boiling water-bath for ten minutes ; 

 a brown or black coloration indicates cottonseed oil. This test- 

 is by no means conclusive of the presence of added cottonseed 

 oil, as the milk of cows fed on large proportions of cotton cake 

 yields butter which will give a brown coloration. 



Halphen's Test for Cottonseed Oil. Gastaldi's Modification. 

 Five c.c. of the fat are mixed in a strong test-tube with 1 drop 

 of pyridine and 4 c.c. of carbon bisulphide containing 1 per 

 cent, of sulphur, the tube is closely stoppered and heated in the 

 water-bath for half an hour. A red colour indicates the presence 

 of cottonseed oil. 



Behaviour of Butter Fat with Solvents. 



Critical Temperature of Solution. Crismer recommends 

 that several drops of the melted and filtered fat be introduced 

 into a small tube 10 millimetres in diameter and 100 to 120 milli- 

 metres long by means of a capillary pipette. An equal volume 

 of alcohol is added and the tube sealed and fastened by a platinum 

 wire to the bulb of a thermometer ; it is then heated in a bath 

 of sulphuric acid till the meniscus separating the two layers 

 becomes a horizontal plane. At this point the thermometer 

 is withdrawn from the bath, and turned sharply two or three 

 times until the liquid becomes homogeneous, after which it is 

 replaced and the temperature allowed to fall slowly, the ther- 

 mometer and tube being constantly shaken. The temperature 

 at which a marked turbidity is produced in the liquid is the 

 critical temperature of dissolution. If absolute alcohol be 

 employed an open tube may be used. 



The alcohol used should have a specific gravity of 0-7967 at 

 15-5 C. ; if the specific gravity differs 0-106 should be added 

 or deducted for each 0-0001 below or above 0-7967. 



When examining butter fat it is necessary to estimate also 



