1882. 139 Trans. N. Y. Ac. Sct. 
vapors is obtained, may be again divided into, first, those where the oil- 
surface is exposed, and called “ open testers,” and, secondly, those where 
the oil-surface is more or less covered, and called “ closed testers.” 
Of these two divisions, the open testers are the first that were made 
and used. About twenty years ago Tagliabue devised his open petro- 
leum-tester. It isa very simple affair, and consists essentially of a glass 
cup to hold the oil, which fits into a vessel of water that can be heated 
slowly. Having filled the glass cup with oil, a thermometer is hung init 
and the whole is placed in the vessel of water ; the latter is heated, and 
a small flame is constantly brought near the oil until a blue flash passes 
over the whole surface ; the thermometer is now noted, and the tem- 
perature at which the blue flash appears is called the “ flashing 
point ” of the oil. 
There are many details that affect the “flashing point” of an oil on 
this apparatus. Among these, I would mention the fact that rapid 
heating of the oil will lower the “ flashing point,” and also that the 
quantity of oil in the cup and the distance of the ignition-flame from 
the oil-surface cause the same result. 
While experimenting with this apparatus, I have noted a phenomenon 
which has a bearing upon the safety of kerosene oil. If the apparatus 
is used in a very quiet room with no air currents, on approaching the 
oil-surface with a flame from time to time, it will be noted that, as the 
oil becomes heated, a temperature is attained at which the flame used 
for ignition becomes perceptibly larger and has a bluish outer envelope, 
which is very distinctly seen. This bluish exterior of the ignition- 
flame is very characteristic, and is noted many degrees below the 
flashing point of the oil in this apparatus. A very little thought will 
show what this phenomenon means. It will be observed that this blue 
extension of the flame becomes more and more marked until the oil 
‘‘flashes.”” It thus appears that, as soon as the flame begins to enlarge, 
the oil is giving off inflammable vapors, but not in sufficient quantity to 
produce a “flash” on the surface. The vapors given off before the oil 
“ flashes” are so light and easily diffusible that they are lost before they 
can form an explosive mixture with the air, for the flash is nothing 
more or less than a miniature explosion upon the oil-surface. Not until 
the vapors become so dense that they cannot diffuse rapidly, will the 
oil flash. The importance of this fact will be apparent later in this 
paper. 
In testing oil with open testers, efforts have been made to produce 
uniform results by filling the oil-cup always to the same level, and also 
by various devices to obtain an ignition-flame of a given size and at a 
given distance from the oil surface. The ignition-flame has been re- 
placed by the electric spark, as in the apparatus of SAYBOLT; but 
