1875] WRITINGS OF JOSEPH HENRY. 479 



were with small lamps, and the comparison in this case, as 

 will be shown, was much against the lard oil. 



Experiments on Lard Oil. 



The first experiment of the new series consisted in charging 

 two small conical lamps of the capacity of about a half pint, 

 one with pure sperm oil and the other with lard oil. These 

 lamps had single-rope wicks, each containing the same num- 

 ber of strands; they were lighted at the same time, and the 

 photometrical power ascertained by the method of shadows. 

 At first the two were nearly equal in brilliancy, but after 

 burning about three hours the flame of the lard oil had de- 

 clined in photometric power to about one-fifth of that of the 

 sperm oil. The question then occurred as to the cause of 

 this decline, and it was suggested that it might be due — first, 

 to a greater specific gravity in the lard oil, which would retard 

 the ascent of it in the wick, after the level of the oil had been 

 reduced by burning in the lamp; or second, to a want of suf- 

 cient attraction between the oil and the wick to furnish the 

 requisite supply as the oil descended in the lamp; or third, 

 it might be due in part to the imperfect liquidity of the oil, 

 which would also militate against its use in mechanical 

 lamps. 



The lard oil was subjected to experiments in regard to 

 each of these points. It was found — by the usual method of 

 weighing equal quantities of the two fluids, that the specific 

 gravity of the lard oil was greater than that of the sperm 

 oil. 



It was also found — by dipping two portions of the same 

 wick into the two liquids and noting the lieight to which 

 each ascended in a given time, that the surface attraction of 

 the sperm oil was greater than that of the lard oil, or in other 

 words the ascensional power of sperm oil was much greater 

 than that of lard oil at ordinary temperatures. This method 

 was also employed in obtaining the relative surface attraction 

 of various other liquids. I say surface attraction instead of 

 capillarity, because it was found in the course of these in- 

 vestigations that substances which had less capillarity (that 



