96 New experimental Researches 
at the end of the first volume of his valuable System, last edition, 
places the boiling point of oil of turpentine at 560°. . Mr, Dal- 
ton, vol. i. p. 39, of his new System of Chemical Philosophy, 
says : “ Several authors have it that oil of turpentine boils at 560°. 
I do not know how the mistake originated, but it boils below 
212°, like the rest. of the essential oils.” I made-with much care 
several experiments on this point, previous to ascertaining the 
force of its vapour, and found its boiling point to be about 316°. 
When recently distilled, however, it will boil at 305°. Did it 
boil below, or even at 212°, as Mr. Dalton asserts, ¢hen, long 
before the included portion in the above experiments had reached 
the 304th degree, it would have acauired such an elasticity as to 
support a high column of mercury, instead of being barely in 
equilibrio with the atmospheric pressure. ' 
Plunge a phial half filled with fresh oil of turpentine into a 
metal cup containing any fixed oil. Heat the cup gradually. 
It will be found that, at the temperature of 316°, the oil remains 
in steady ebullition, as indicated by a thermometer suspended in 
the centre of the phial. Prior to this, even at 212°, some small 
bubbles will be evolved, principally owing to the moisture di- 
spersed in the pores of the oil, from the water originally mixed 
with the crude turpentine in its distillation. If the heat be very 
rapidly thrown in, while the upper surface of the oil of turpen- 
tine has the area only of a one or two ounce phial, it is possible 
to heat it to 360° or 370°, in apparent contradiction to the theory 
of latent heat; for when a liquid boils in an open vessel, ac- 
cording to Dr. Black, its temperature should remain stationary. 
The true cause of this phenomenon is developed towards the 
conclusion of this memoir. The specific caloric of the vapour of 
the volatile oil is so small, compared to that of water, that the 
heat may readily be quicker introduced than the boiling process 
can abstract it. Concerning the boiling point of this oil, I have 
since inquired of a manufacturer ; and he states its boiling point 
at 320°. Essential oil of rosemary, when kept for some time, 
boils at 270°; recent oil at 212°. To assign the cause of this 
difference, is foreign to our present object. 
The vapour of ether follows nearly the same rate of expansion 
as water, if we start from their respective boiling points. This 
was observed also in Mr. Dalton’s experiments; and from this 
single analogy, chiefly, he laid down the general law, ‘ that the 
variation in the force of vapour from all liquids is the same ‘for 
_-the same variation of temperature, reckoning from vapour of any 
given force.”’ 
My experiments on oil of turpentine and petroleum show the 
fallacy of this generalization, if we reckon the common thermo- 
metric ‘scale a tolerably correct index of temperature 5 but ‘if, 
with 
