INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 487 



3. Freedom from acids or alkalies. 



4. Eesistance to freezing. 



5. Actual burning in fifth-order lamps for at least ten hours. 



G. Photometric power afterburning one hour, and again after burning 

 ten hours. 



7. The condition of the wick at the end of the burning. 



These tests are of very unequal value, and several of them might be 

 dispensed with, were others reduced to an absolute standard determined 

 by the actual experience of burning in the light-houses. 



The specific gravity of impure lard and of that which has been care- 

 fully refined differ but little, and hence, unless the experiment be made 

 by means of a delicate balance, the indications will be of comparatively 

 little value. Still, as a given samjile might contain some foreign sub- 

 stance which is not usually mixed with this oil, the test with the hydrom- 

 eter should not be omitted. 



In making this test, a cylindrical vessel containing the oil, of suffi- 

 cient diameter to jiermit the hydrometer to float freely without hinder- 

 ance from the sides, should be immersed in a vessel containing several 

 gallons of water, which, when once reduced to G0<^ 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 neces- 

 sary 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 experi- 

 ment 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 considera- 

 ble, the difference between different samj^les 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 alkilies is easily made, and should in no 

 case be omitted. A portion is put into beaker-glasses, with a slip of lit- 

 mus-paper in one and a slip of tumeric paper 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 brown- 

 ness, the oil may be considered void of free acid and of alkali, both of 

 which would lessen its value, the former tending to corrode the lamp 

 and the latter interfering with its burning quality. 



Resistance to freezing is an important test, but 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 temperature by being plunged in a 



