1875] WRITINGS OF JOSEPH HENRY. 481 



the sperm oil. These results were recognized as having an 

 important bearing on the question of the application of 

 lard oil as a light-house illuminant. It only required to 

 be burned at a high temperature, and as this could be read- 

 ily obtained in the case of larger lamps, there appeared to 

 be no difficulty in its application. 



The previous trials had been with small lamps with single 

 solid wicks, instead of the Fresnel lamp with hollow burn- 

 ers. After these preliminary experiments two light-houses 

 of the first order at Cape Ann, Massachusetts, (separated by 

 a distance of only 900 feet,) were selected as affording excel- 

 lent facilities for trying — in actual burning, the correctness 

 of the conclusions which had been arrived at. One of these 

 light-houses was supplied with sperm oil and the other with 

 lard oil, each lamp being so trimmed as to exhibit its great- 

 est capacity. It was found by photometrical trial that the 

 lamp supplied with lard oil exceeded in intensity of light 

 that of the one furnished with sperm oil. The experiment 

 was continued for several months, and the relative volume 

 of the two materials carefully observed. The quantity of 

 sperm oil burned during the continuance of the experiment 

 was to that of lard oil as 100 is to 104. 



The freezing temperature of lard oil depends upon the 

 temperature at which it was expelled by pressure from the 

 animal tissues in which it was contained. It is higher how- 

 ever than the freezing temperature of sperm oil, on an aver- 

 age of from 3° to 4° F., but this is a matter of no practical 

 objection to the substitution of lard oil for sperm oil, since 

 the heat evolved from an Argand lamp is — in cases where 

 the draught passes through the reservoir, sufficient to keep 

 the lard oil liquid even during the lowest external tempera- 

 ture. Indeed, the small difference in temperature in freez- 

 ing of the two oils is a matter of little moment in cases 

 which frequently happen when the temperature of the at- 

 mosphere is below zero on the Fahrenheit scale. At such 

 a temperature both oils would alike become solid unless 

 some means were afforded for preventing the freezing. 



The next step toward the introduction of lard oil was the 



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