304 CONSIDERATION OF CARBON COMPOUDDS. 



572 F.), or even higher. The crude oil is purified, by treating it 

 with sulphuric acid, followed by other processes of refining, and 

 finally by fractional distillation, in order to separate the members of 

 low boiling-points from those of higher boiling-points. 



The hydrocarbons of low boiling-points, chiefly a mixture of C 5 H 12 

 and C 6 H 14 , are official, under the name of benzin or petroleum-ether, 

 which name must not be confounded with benzene or benzol, C 6 H 6 . 

 According to the U. S. P., benzin should have a specific gravity from 

 0.67 to 0.675, and a boiling-point of 50 to 60 C. (122 to 140F.). 



Other similar liquids are sold in the market under the name of 

 rhigoline, B. P. about 2rC. (70 F.) and gasoline, B. P. about 75 

 . (167 F.) ; they are highly inflammable. 



The paraffins distilling between 150 and 250 C. (302 and 408 

 F.) constitute the common illuminating oil, various kinds of which are 

 sold as kerosene, paraffin oil, astral oil, mineral sperm oil, etc. The 

 danger which arises in the use of coal-oil as an illuminating agent 

 is caused by the use of oils which have not been sufficiently freed 

 from the more volatile members of the series, which, when but 

 slightly heated (or even at ordinary temperature) will vaporize, and 

 upon mixing with atmospheric air, form explosive mixtures. An oil 

 to be safely used for illuminating purposes in common lamps should 

 not give off inflammable vapors (or flash) below 49 C. (120 F.). 



Experiment 41. Various forms of apparatus are used for the exact determi- 

 nation of the flashing-point; students may determine it approximately by 

 operating as follows : Fill a cylinder (about one inch in diameter and six 

 inches high) two-thirds with coal-oil, suspend in the oil a thermometer, place 

 the cylinder in a vessel with water (water-bath), keeping the level of the oil 

 even with that of the water, and heat the latter slowly. Cover the cylinder 

 loosely with a piece of pasteboard, and when the thermometer indicates a rise 

 in temperature pass a small flame quickly over the mouth of the cylinder after 

 having removed the pasteboard. Repeat this operation, from degree to degree, 

 until a bluish flame is noticed running down to the surface of the oil. The 

 temperature at which this takes place indicates the flashing-point. 



After the illuminating oil has been distilled off, a mixture of sub- 

 stances passes over, which is used for lubricating purposes or furnishes, 

 after having been purified by treatment with bone-black, the official 

 articles known as soft and hard petrolatum. Both are fat-like masses, 

 more or less fluorescent, varying in color from white to yellowish or 

 yellow, and almost without odor or taste. The article sold as vaseline 

 is practically identical with soft petrolatum. 



Liquid petrolatum of the U. S. P. is a petroleum of a specific 

 gravity 0.875 to 0.945. 



