482 WRITINGS OF JOSEPH HENRY. [1875 



devising of a system by which it could be inspected, and the 

 Board assured — before it should be too late to remedy the 

 evil, that the lard oil purchased was of a good quality. This 

 was a matter of great importance, and involved no small 

 degree of responsibility, since the contractor was entitled 

 to his pay immediately after the acceptance of the oil ; 

 and the quantity purchased amounted annually to nearly 

 100,000 gallons. 



The conclusion was arrived at that it was impossible — from 

 any single test that could be applied to small samples, to 

 determine the quality of the oil as applicable to light-house 

 purposes, and that in the present state of our knowledge as 

 to its character, the following tests are required to fully in- 

 sure in all cases the required quality of the article; 



1. Specific gravity at 60° F. 



2. Liquidity at different temperatures. 



3. Freedom from acids or alkalies. 



4. Resistance to freezing. 



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

 hours. 



6. Photometric power after burning one hour, and again 

 after burning ten hours. 



7. The coadition 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 abso- 

 lute standard — determined by the actual experience of burn- 

 ing in the light-houses. 



The specific gravity of impure lard oil and of that which 

 has been carefully 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 sample might contain some foreign substance 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 sufficient diameter to permit the hydrometer to float 

 freely without hindrance from the sides, should be immersed 

 in a vessel containing several gallons of water, which when 



