INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 503 



perature. If the rise of the temperature be very sudden, the thermom- 

 eter will uot respond, and the real flashing temperature will be h\'^]wv 

 than that which is indicated. 



The next test is that of llriug of the mass of the liquid, which is some- 

 times 10 or 12 degrees higher than that of the flashing temperature, but 

 generally the two are very near each other. 



The next test is the determination of the specific gravity. This was 

 obtained by weighing, in a glass flask Avith a narrow neck, an e(iual 

 quantity of distilled water and of the oil in question; the ratio of the 

 two, reduced to water as unity, gave the specific gravity required. To 

 facilitate the operation, a flask, containing Just 1,000 grains of distilled 

 water, was balanced by a permanent weight. The scales were tested 

 by double weighing. The first series of weighing was made at the tem- 

 perature of 74° F., that of the apartment in which the exi)eriment was 

 conducted; but oil and other substances change their bulk, and conse- 

 quently their specific gravity, with a change of temperature. It is 

 therefore necessary, in order that results may be compared, that the - 

 experiments be all made at the same temperature, or reduced to a stan- 

 dard temperature. The temperature formerly adopted in England for 

 specific gravity is 02° F.; but in the case of petroleum, the temperature 

 of 00° has been adopted in this country and England. In the first series 

 of experiments made with the oils in question, the weighing was con- 

 ducted at a temperature of H^, as we have said, namely, that of the 

 atmosphere at the time. A series of experiments at a lower temper- 

 ature was afterward made, in order to obtain a correction by which to 

 reduce the specific gravity first obtained to that of a temperature of 00° ; 

 but as each oil exhibits a diflerent rate of expansion by heat, the process 

 became very laborious. Experiments were therefore made to determine 

 the correctness of obtaining the specific gravity of the oils by means of 

 a hydrometer. This was found to differ from that obtained by weigh- 

 ing within one per cent., and was therefore concluded to be sufliciently 

 accurate for practical purposes. 



To obtain the specific gravity of the oils by means of a hydrometer, 

 a vessel containing, say, 10 gallons of water, of a depth of about 14 

 inches, is provided ; into this are introduced several glass cylinders con- 

 taining the oil, and into these cylinders the hydrometers are plunged, 

 the level of the oil being so far above the water that the under contact 

 of the surface of the liquid with the scale may be observed. Before 

 inserting the glass cylinders containing the oils into this water-bath, 

 the liquid is brought, by mixing ice-water with it, to the temperature of 

 60°, at which temperature it may be kept for a long time, on account of 

 the large quantity of the liquid and the great specific heat of the water. 

 A change of temperature may be prevented by occasionally adding a 

 small quantity of ice-cold water, care being taken to mingle the mixture 

 by stirring. By this process may be obtained the specific gravity at GO^ 

 of a large number of samples in a comparatively short time. In this 



