500 WRITINGS OF JOSEPH HENRY. [1875 



for cooling it ; the still is then gradually heated ; the first 

 product that passes over is gaseous at ordinary temperatures, 

 and can only be condensed into a liquid form by cooling the 

 worm with ice, or by compressing the gas with an air-pump 

 into a strong receiver. After all the vapor is given oif at 

 the temperature, say of 90° F., the temperature of the liquid 

 in the still is raised, and it then exhales a vapor at a higher 

 temperature and of greater density; and thus on successively, 

 a series of liquids is produced, each of which requires to be 

 heated to a higher degree before taking fire on the approach 

 of a lighted match. The more volatile vapors are heavier 

 than atmospheric air, and when suffered to escape from the 

 cask containing them will flow along the surface of the floor 

 of a room, and reaching a distant fire-place will ignite, and 

 burning backward to the reservoir will set fire to the oil 

 from which they emanated. 



Many serious accidents have occurred in this way, by the 

 firing of a canister containing petroleum oil which has been 

 left open, although at a distance in some cases of from 20 to 

 30 feet from a lighted fire. Another source of danger from 

 the lighted oils from which the more volatile vapors arise — 

 results from the fact that these vapors when mixed with a cer- 

 tain portion of atmospheric air explode on the approach of a 

 flame with extreme violence. When the proportions of 

 vapor and air are equal no explosion takes place; but when 

 they are in the ratio of 10 parts of the vapor in volume to 

 100 parts of air the explosion is most violent; when the 

 quantity of air or of petroleum vapor is increased or dimin- 

 ished, the explosion is less violent until one or other becomes 

 excessive, and when the vapor is in excess, it kindles with- 

 out explosion, as is the case with ordinary street gas when 

 issuing from the burner. 



A notable case of the explosive quality of a mixture of 

 petroleum vapor and air occurred in connection with the 

 light-house service in 1864, on Lake Michigan. The keeper 

 in one of the light-houses of this district substituted on his 

 own responsibility an ordinary kerosene lamp of tinned 

 iron for the usual lard oil lamp. This gave a good light 



