504 INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 



country and England the density or relative weight of petroleum oils is 

 generally expressed in terms of the arbitrary scale of Beaume's, instead 

 of that of the specific gravity. The following table gives the equiva- 

 lent of the Beaume scale in terms of specific gravity: 



Beaumeh hydrometer for liquids lighter than water. 



Another test to which the mineral oil was subjected was that of a 

 reduction of temperature. For this purpose the samples were placed 

 in an air-bath reduced to the temperature of 25° F. At this tempera- 

 ture several of the oils exhibited a thickened condition, especially tbose 

 of the higher fire-test. The apparatus used for this purpose was the 

 same as that previously described as employed in the case of lard-oil. 



The next test to which the oil was subjected was that of its liquidity. 

 This test is of some importance in regard to lamps in which the oil is 

 pumped \\\} by machinery, and also as to the solid matter in the oil. It 

 therefore gives a characteristic of the oil which with others serves to 

 determine its degree of impurity. For this purj^ose the same method 

 was employed as that described for determining the liquidity of lard- 

 oil. The liquidity exhibited by this process was very different in differ- 

 ent oils. 



All the experiments on the flowing of the oils were made at the tem- 

 perature of the air, which was from 72° to 74°. In this case, as with 

 lard, a marked difference was found in the time of flowing at dift'erent 

 temperatures, and hence for comparison the experiments should be made 

 at a standard temperature. 



Another experiment was made to ascertain whether oils of higher 

 flashing test gave off a vapor at the ordinary temperature of the atmos- 

 sphere; for example, at about 70^. For this i)uri)Ose a barometer tube 

 of about 33 inches in length, and an interior diameter of one-half of an 

 inch, was filled with warm mercury inverted in a basin of the same 

 metal. The finger was then placed under the open mouth of the tube 

 in the basin and the tube slowly inverted so as gradually to pass the 



