484 WRITINGS OF JOSEPH HENRY. [1875 



glass, in the side of a cylindrical wooden tub about 2^ feet in 

 diameter. Concentric within this tub was placed another 

 cylindrical vessel (of smaller diameter) of zinc, filled with 

 a freezinty-mixture of salt and pounded ice. A series ol 

 small beaker-glasses, filled with the several samples of oil, 

 was placed opposite the windows in the space between the 

 two cylinders, each containing a thermometer which could 

 be read through the window. The whole was then inclosed 

 by a tightly-fitting cover, through which projected the 

 handle of a crank, by which the freezing-mixture could be 

 stirred. The samples of the oil subjected to this cold-air 

 bath gradually passed through the several stages of diminu- 

 tion in limpidity and clearness — to opacity and solidity, the 

 time of each being noted. 



The most reliable test is that of actual burning in a lamp 

 of the fifth order and the measurement of the pliotometri- 

 cal power. The objection to the application of this test to 

 the oil of every barrel is the large quantity of oil required 

 and the amount of labor involved in the proper execution 

 of the process. Thus in testing 60,000 gallons contained 

 in casks of forty gallons each, at least 500 gallons would be 

 required. It is therefore evident that this test can only be 

 applied to samples selected from a given lot, while the single 

 barrels are proved to be of a similar character by the more 

 simple tests. 



Another method of insuring that all the casks of a given 

 lot contain oil of the same quality consists in taking a 

 small equal portion from each of several casks and min- 

 gling them together, the quality of the compound being 

 ascertained by the application of burning or the other tests. 



The determination of the photometrical power is in the 

 present state of science (unless great precaution is observed), a 

 problem of some uncertainty. The difficulties are of two 

 kinds, the first to find a photometer which shall give the ratio 

 of the two lights, and second, to find an invariable standard 

 to which oil of the proper quality may always be referred. 



These difficulties can I think be sufficiently overcome 

 for the practical purposes of the Light-House Board. The 

 greater difficulty is that of obtaining a standard of reference. 



