346 On the Igniting Point of Petroleum. 
liquid until the tube is half full. Stir the liquid well with a 
naked thermometer, having the usual degrees marked on 
the stem, shaking also so as to keep the upper part of the 
tube well wetted with the liquid, and note the temperature. 
Now introduce a flame (of a thin splint of wood, or, far 
better, a small gas flame, a quarter of an eighth of an 
inch long) into the mouth of the tube to within half an inch 
of the surface of the liquid, quickly withdrawing it, and 
noticing whether a thin blue flame runs between the test- 
flame and the surface of the oil. If not, warm the tube by 
passing the bottom of it gently through a spirit lamp, or 
other flame, or by dipping the lower portion of the tube 
into hot water, constantly stirring the liquid with the ther- 
mometer, frequently noting the temperature, and intro- 
ducing the test-flame every minute or so. The temperature 
at which the thin blue flame appears will be the igniting- 
point of the petroleum, the point at which it gives off in- 
flammable vapour. To correct this result, let the tube 
gradually cool, introducing the test-flame as before. The 
lowest temperature at which the vapour takes fire is the 
true igniting-point. To ascertain the specific gravity, pour 
the petroleum or paraffin oil in the test-tube until the latter 
is about three-fourths full; insert a thermometer, and warm 
or cool until the temperature is about 60 degrees;* now 
immerse the hydrometer, and take care that it fairly floats 
in the liquid ; the point on the stem of the hydrometer, cut 
by the under surface of the liquid, will be the specific 
gravity. A copy of these directions will be found in the 
case, as sold by Mr. Casella. 
This particular hydrometer might be termed a Petro- 
leometer ; that name would, however, perhaps best desig- 
nate the whole box or set of articles. The most accurate 
method of taking gravities is of course the specific-gravity 
bottle; but a hydrometer, if well made, gives, I find, num- 
bers varying not more than one degree from those of the 
bottle, whilst its use involves far less trouble and expense. 
As an indication of the extent to which confidence can 
be placed in an igniting-point of petroleum, taken in the 
manner recommended, I may state that two different ob- 
servers, experimenting at different times on three different 
* Five degrees of temperature make about one degree differ- 
ence of specific gravity. At 70° Fahr. an oil will be about two 
degrees lighter in specific gravity than at 60° Fahr., and at 50 
Fahr. two degrees heavier. 
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