510 WRITINGS OF JOSEPH HENRY. [1875 



happened in the photometric room in the case of a lamp of 

 the fourth order under trial ; the heat unsoldered an air-tube 

 and let down the oil from the reservoir on the flame, which 

 produced so fierce a combustion that it would have set fire 

 to the building had it not been of fire-proof materials. 



The gradual introduction of mineral oil will be made 

 as rapidly as experience indicates the best and safest 

 mode of employing it. It has already been adopted in the 

 smaller lamps for lighting the Mississippi and its principal 

 tributaries. The substitution however is not on account of 

 the superior quality of this oil in comparison with lard oil, 

 — since we think the latter as an illuminating material is 

 inferior to no other at present in use, — but simply on account 

 of the comparative cost of the two materials. This relative 

 expense will be definitely ascertained after we have deter- 

 mined the best form of lamps to be used. Experiments thus 

 far have been principally confined to the lower orders of 

 lamps. 



