1875] WRITINGS OP JOSEPH HENRY. 483 



once reduced to 60° by the addition of ice-cold water, 

 can (on account of the great specific heat of water,) be readily 

 kept at that temperature by a slight addition of cold water 

 from time to time, the whole being continually stirred. It 

 is scarcely necessary to state that the vessel containing the 

 oil must be so weighted at the bottom that it will stand erect 

 in the cold bath in which the experiment is made. 



Liquidity at different temperatures is a test similar in 

 character to that of specific gravity ; although the difference 

 in degree of liquidity of different kinds of oil, such as sperm, 

 whale, and lard, is very considerable, the difference between 

 diff"erent samples of lard oil is small. Still this test (for a 

 similar reason to that given for the specific gravity,) should 

 be applied. 



The test for free acids and alkalies is easily made, and 

 should in no case be omitted. A portion is put into beaker- 

 glasses, with a slip of litmus-paper in one and a slip of 

 turmericpaper in the other, and suffered to remain immersed 

 perhaps twenty -four hours; and at the end of that time, if 

 one of these papers exhibits no redness and the other no 

 brownness, the oil may be considered void of free acid and 

 of alkali, — either of which would lessen its value, the former 

 tending to corrode the lamp, and the latter interfering with 

 the burning quality. 



Resistance to freezing is an important test, l)ut not as 

 easily applied in the case of lard-oil as might at first be 

 imagined. Lard-oil possesses the remarkable property of 

 resisting the influence of a low temperature if suddenly 

 applied, while it will freeze at a much higher temperature if 

 the cold be continued for several hours. 



For example if a small portion of lard oil be placed in a 

 test-tube and submitted to a rapid diminution of tempera- 

 ture by being plunged in a freezing-mixture, it will remain 

 liquid for some time at a temperature of 19° or 20°, whereas 

 it will congeal at a temperature of 40° if suffered to remain 

 at that temperature for several hours. 



The plan adopted for determining the freezing-point of 

 different samples of oil, at one operation, consisted in 

 making a series of small openings or windows, closed with 



