488 INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 



freezing-mixture, it will remain liquid 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 sam- 

 ples of oil at one operation consisted in making a series of small open- 

 ings, or windows, closed with 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 

 freezing-mixture of salt and pounded ice. A series of small beaker- 

 glasses, filled with the several sami^les of oil, were placed opposite the 

 windows in the space between the two cylinders, each containing a ther- 

 mometer 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 i)ass through 

 the several stages of a diminution 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 photometrical power. The objection 

 to the application of this test to the oil of every barrel is the large quan- 

 tity of oil required and the amount of labor involved in the x)roper exe- 

 cution 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 mingling them together ; the quality of the 

 compound being ascertained by the application of burning or the other 

 tests. 



The determination of the photometrical i^ower is, in the present state 

 of science, unless precaution is observed, a problem of some uncertainty. 

 The difficulty is of two kinds, the first to find a photometer which shall 

 give the ratio of the two lights, and, second, to find an invariable stand- 

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

 difiiculties, I think, can be sufficiently overcome for the practical pur- 

 poses of the Light-House Board. The greater difficulty is that of ob- 

 taining a standard of reference. For this a samijle of lard-oil manu- 

 factured by Mr. Alden, of Boston, was at first employed, but this, itself, 

 was- found to be variable, and hence we were obliged to adopt some 

 other standard. The one which has been finally adopted is English 

 sperm cand les, which burn with considerable uniformity at the rate of 

 120 grains per hour, or two grains j)er minute. 



In regard to the investigation, the experiments were carried on under 

 many difficulties. They were made at first in the engineer's office of the 



