502 INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 



an illuminating material, as soon as oil could be obtained in this country 

 of a suitable character, lard-oil having advanced in price to such a de- 

 gree as to render this change desirable in an economical j)oint of view. 

 In the mean time experiments had also been made in France and 

 England for the purpose of introducing mineral oil as a light-house illu- 

 minant,. but it was not until 1873 or 1874 that the result was entirely 

 satisfactory. 



The process of manufacturing the oil has been very much improved 

 in this country of late years, and there are now several companies which 

 profess to produce oil entirely safe, and otherwise suitable for light- 

 house purposes. In view of further experiments with mineral oil, an 

 advertisement was inserted in the papers, in 1874, requesting man- 

 ufacturers to send samples of their oils to be tested at the Light-House 

 depot at Staten Island, and in accordance with this a number of speci- 

 mens were received and submitted to examination. 



The first test to which the oils thus furnished were submitted was 

 that of flashing, that is, the determination of the temperature at which 

 the oil gives oft' a vapor which will flash into a flame on the approach of 

 a small taper, or, in other words, which indicates the rise of a vapor 

 which, mixed with atmospheric air, will tend to produce an explosion. 

 The flashing temperature differs however from that at which the liquid 

 takes fire as a whole. This will be understood if we suppose that two 

 liquids have been mixed together, a light and a heavy one ; the flash in 

 this case will be due to the vapor from the lighter mixture, while the 

 burning is due to the temi)erature at which the compound is fired. To 

 make this flashing test requh^es considerable precautions. First, the oil 

 to be tried is gradually heated by a spirit-lamp in a water-bath, a sensi- 

 tive thermometer being suspended in the oil with the bulb slightly be- 

 low the surface, the heat of the water is very slowly increased by re- 

 moving from time to time the spirit-lamp from under the basin of the 

 water-bath which contains the oil, and the point of flashing is obtained 

 by passing over the surface of the oil a small flame until the first indi- 

 cation of flash is observed. The flame should not be so large as to heat 

 the surface, and is best produced by a very small jet of gas from a glass 

 tube drawn nearly to a point and connected with the gas pipe of the 

 house by a tube of India-rubber, the quantity of gas being regulated by 

 a stop-cock, so that the flame is a mere pencil of light about a quarter 

 of an inch in length and a twentieth in diameter. The basin which 

 contains the oil is about four inches in diameter, and is sometimes cov- 

 ered with a plate of thin glass, the thermometer passing through an ap- 

 erture in this cover, and a larger hole being left open in the same for 

 inserting the pencil of the flame. The basin containing the oil is some, 

 times left entirely open, the cover being discarded, but we do not think 

 this as safe a method as the other. Great caution must be taken in 

 raising the temperature very gradually, so that every part of the liquid 

 may have the same heat and the thermometer thus truly indicate the tern- 



