TOt. XVIII.] PHILOSOVHICAL TRANSACTIONS. ggf 



the oil has in producing this fire, whether it be only a pabulum, or fuel, for 

 the spirit to actuate, and so be merely passive ? or whether it contributes any 

 particles that help to excite this flame ? In order to the resolving this doubt, 

 we consider, that these essential oils are produced from seeds that have very 

 active or warm parts or spirits, such as will easily ferment and heat. And 

 besides the potential heat of the vegetable, essential oils contain a volatile salt, 

 which gives much of that pungency to the taste. So that our oil is not a bare 

 pabulum ignis, or an unactive principle, but on a double account, both on the 

 score of the incalescent oil and of the inherent salt, conspire with the com- 

 pound spirit to make this great heat, explosion, and accension. 



In the catalogue of experiments we may further observe, that of the light 

 essential oils drawn from seeds of vegetables, all of them make a great ebulli- 

 tion with an explosion, but that few of them do actually take fire, and that all 

 of those that are extracted from trunks, or other parts of our vegetables, do 

 certainly take fire and flame. Wherefore, having observed that those which do 

 not take fire or flame, do yet make as great an explosion and ebullition, and 

 probably as great a heat as those that did, I was apt to impute it to the light- 

 ness and too great subtilty and volatility of those essential oils, whose very ac- 

 tive particles too soon exhale or fly away. And this conjecture seems to be 

 justified by the addition of a more grumous body, as balsam of sulphur made 

 with oil of turpentine, to our most volatile or subtile oils, which then produce 

 a flame, whose particles being grosser, or more grumous, will detain the more 

 volatile oil from too sudden an explosion, and give more time to the fiery spirit 

 to penetrate, and mix itself with those combustible materials. And this may 

 be one reason why the ponderous oils distilled from the roots or ligneous parts 

 of a plant do all take fire, viz. because the parts of this sort of oil, lying closer 

 together, do not so soon dissipate after the spirit is thrown upon it. And then 

 as to the specific gravity, the difference is also very considerable, which any one 

 may find by this familiar way ; if you fill a glass with 1 ounce of the essential 

 oil of the seeds, you will require Q drams of the ponderous oil of the vegetable 

 to fill up the same space. This is also obvious to any spectator, that most of 

 these oils thus distilled are more ponderous than common water, by their sink- 

 ing to the bottom ; whereas all our essential oils, extracted from the seedy parts, 

 swim on water, and some are lighter than the best rectified spirit of wine, but 

 most are lighter then brandy, which has made the chemists call them ethereal 

 oils. 



The ponderous oils have yet one advantage over the lighter volatile oils ; for 

 being exposed to a longer and greater degree of fire than the others, they unite 



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