1875] WRITINGS OF JOSEPH HENRY. 499 



light-house illuminant was required. It is therefore with 

 regret that we are urged, on account of the increased price 

 of the article, due in some degree to the reputation as a 

 burning material given it by the board itself, to substitute 

 for it a less reliable but a much more economical material. 



Experiments on Mineral Oils. — At the time lard oil was in- 

 troduced, a series of experiments was made on the compara- 

 tive value of the different petroleum oils used in this country. 

 They were all considered too dangerous to be intrusted to the 

 ordinary keepers of the light-stations of our coast. Since the 

 date of these investigations however, improvements have 

 been made in the manufacture of these oils, by which a much 

 greater range has been obtained in the temperature at 

 which they give off a noticeable vapor. During the last 

 two years, a new series of investigations therefore has been 

 made relative to these illuminating agents, of which we pro- 

 pose in the succeeding pages to give a brief account. 



The crude petroleums of the Pennsylvania oil region are 

 of a greenish or yellowish appearance, and have a specific 

 gravity of 45° to 49° Beaume at a temperature of 60° Fahren- 

 heit. Some are so volatile as to evaporate rapidly at the 

 ordinary temperature of the air, rendering it dangerous 

 to approach an open cask of crude petroleum with a flame ; 

 others are much less volatile, requiring a temperature of 

 from 200° to 300° F. to vaporize them. The volatility of 

 the hydro-carbons is intimately connected with their specific 

 gravity. They become heavier as the volatile ingredients 

 are driven off by heat. The inflammability of the oils is 

 also connected with their volatility and specific gravity. 

 The light volatile oils ignite at ordinary temperatures, as we 

 have said, on the approach of a burning match, while the 

 heavier require a higher temperature for ignition. The 

 process of manufacturing these oils consists in separating 

 them from each other as they occur in the crude oil of the 

 springs by what is called fractional distillation ; for this 

 purpose the crude oil is placed in an iron still provided 

 with a worm of the same metal submerged in a tank of water 



