RES 



pod'efs fimilar properties. The oils termed drying, fuch as 

 linfeed and nut-oil, by the prefence of oxygen, affume a 

 folid form, unlike the greafy fubftance afforded by the lat 

 oils, and more like horn, or elaflic gum (caoutchouc.) 

 The volatile oils are fufceptible of the fame change, from 

 liquidity to folidity, by the agency of oxygen ; the refult 

 being refins, which differ from each other in nothing ma- 

 terial, excepting their degrees of folidity, and their peculiar 

 odour. If we are to conclude that every effential or volatile 

 oil is capable of being converted into a refin, which is 

 highly probable, the number of refins will be very great. 

 Indeed the known fpecies are vaftly too numerous for our 

 notice in this place : we (ball, therefore, confine our atten- 

 tion to a few of the moft particular refins, efpecially thofe 

 which are valuable in the arts, and in medicine. Dr. 

 Thomfon gives the following lift of refins, with the names 

 of the vegetables from which they are obtained. 



i. Turpentine of Chio, from the piftacia terebinth us. 



2. Venice turpentine. 



3. Strafburgh turpentine, from the common fir. 

 4.. Pitch, from the pinus picea. 



5. Elemi, from the amyris elemifera. 



6. Maftic, from the piftacia lentifcus. 



7. Sandarach, from the juniper. 



8. Guaiac, from the guaiacum officinale. 



9. Laudanum, from the ciftus ladaniferus. 



10. Dragon's blood, from the draexna draco. 



1 1. Balfam of Mecca, from the amyris opobalfamum. 



12. Balfam of Copahu, from the copaifera officinalis. 

 To thefe may be added, 



13. Balfam of Canada, from the pinus balfamea. 



14. Guaiacum, from the lignum vitoe. 



15. Copal, from the copallinum of North America. 

 Some of the refins are combined with benzoic acid, by 



which their properties are altered : of thefe are benzoin, bal- 

 fam of Tolu, balfam of Peru, and itorax. 



Another clafs of thefe bodies are combined with gum, 

 and hence have been called gum refins : of thefe are oli- 

 banum, galbanum, fcammony, allafcetida, myrrh, ammoniac, 

 aloes, and opium. They admit of analyfis, either by fepa- 

 rating the refinous part by alcohol, or the gummy part by 

 water. 



Thefe latter fubftances are ufed in medicine. Some of 

 the firft clafs, or the pure refins, are ufed as cements ; and 

 their folution in alcohol conftitutes a variety of ufeful var- 

 nifhes. 



Thofe refins which are diftinguifhed by the name of bal- 

 fams are faid to be fuch as contain benzoic acid. It ap- 

 pears, however, they have alfo been diftingufhed for their 

 liquidity, and from their excefs of odour ; fo that at pre- 

 fent the phrafe appears to be very indefinite. See Balsam. 

 The refins are of various degrees of confidence and hard- 

 nefs. The balfams of Canada and Mecca are thickifh fluids : 

 the former is often fo liquid as to be fpread with a brufh, 

 and is fometimes uled for varnifhing pictures. It gradually 

 hardens, on expofure to the air, and ultimately may be 

 rubbed with the hand without foiling. 



Others of the refins become hard with very little expofure 

 to the air, and arc found in hard tears, flicking to the tree 

 which affords it : of thefe are maftic, fandarach, and copal. 

 Thofe which do not eafily become folid may be obtained in 

 that form, by diltilling off the thin part. Refin and pitch 

 are obtained in this way : the former is obtained by dif- 

 tilling turpentine, the latter by dillilling tar. Tar is no 

 other than turpentine contaminated with the foot, which is 

 produced by the partial combuftion employed for its ex- 

 traction from the wood. 



R E 8 



All the refins become harder by expofure to a moderate 

 heat. It is upon this fac~t that the art of the japanner de- 

 pends. If the furface to be japanned be covered with com- 

 mon tar only, and expofed to the temperature of 300 for 

 a length of time, the coating becomes hard and infufible. 

 At the fame temperature, any other reiin applied in the 

 fame way, would affume a fimilar hardnefs. 



Some animals and minerals, as well as the vegetables, 

 afford refins, or bodies very analogous to them. The con- 

 crete part of the bile of animals has moft of the properties 

 of a reiin, and fhell-lac is furniffied from the infeft called 

 coccus lacca. 



Ear-wax is alfo found to have the properties of a refin ; 

 and the fubftances well known as perfumes, caftor, civet, 

 and mufk, are no other than animal refins. For the par- 

 ticular application of refins to varnifiing and japanning, fee 

 thofe articles. 



Refins are employed for many purpofes. Thofe of the 

 cheapeft kind are ufed for torches, and to cover the outfides 

 of fhips and boats. The fine tranfparent refins compofe 

 varnifhes. Many of them are employed medicinally : fuch 

 are thofe which enter into the compofition of ointments and 

 plafters ; or internally, as the refins of fcammony, jalap, and 

 turpeth, which are purgative. Other refins, the fmell of 

 which is agreeable, as benjamin and ftorax, are employed as 

 perfumes. 



Resin, Elaflic. See Caoutchouc. 

 Resin, Maftic. See Mastic. 



RESISTANCE, or Resisting Force, in Phyfics, any 

 power which afts in oppofition to another, fo as to deftroy 

 or diminifh its effeft. 



Of refiftance there are various kinds, arifing from the va- 

 rious natures and properties of the refifting bodies, and go- 

 verned by various laws : as the refiftance of folids, the 

 refiftance of fluids, the refiftance of the air, &c. 



Resistance of Solids, in Mechanics, is the force with 

 which the quiefcent parts of folid bodies oppofe the motion 

 of others contiguous to them. 



Of this there are two kinds. The firft, where the refift- 

 ing, and the refilted parts, i. e. the moving and quiefcent 

 bodies, are only contiguous, and do not cohere ; i. e. where 

 they conftitute feparate bodies or maffes. 



This is what M. Leibnitz calls refjlance of the furface ; 

 but which is now more commonly denominated friftion ; 

 for the laws of which, fee Fiuction. 



The fecond cafe of refiftance is where the refifting and 

 refilled parts are not only contiguous, but cohere ; i. c. are 

 parts of the fame continued body or mafs ; for the pheno- 

 mena and laws of which, fee Cohesion. 



To which wc may alfo add, the refiftance which takes 

 place between furfaces of folids, when completely in con- 

 tact, though not forming one and the fame body, or the 

 refiftance they offer to reparation, the particulars of which 

 are treated of under the article Adhesion. 



RESISTANCE of the Fibres of folid Bodies, Theory of the. 

 To form an idea of this refiftance or renitency of tin- parts, 

 fuppofe a cylindrical body fufpended vertically by one end. 

 Here all its parts, being heavy, tend downwards, and en- 

 deavour to feparate the two contiguous planes, where the 

 body is the wcakcll ; but all the parts relilt this feparation 

 by the force with which they cohere, or are bound to- 

 gether. Here then are two oppofite powers; viz. the 

 weight of the cylinder, which tends to break it ; and the 

 force of cohefion of the parts, which refifts the frachirr. 



If the bafe of the cylinder be increafed without incrcaling 

 its length, it is evident the refiftance will In- increafed in tin- 

 fame ratio as the bafe ; but the weight alfo increaks in the 

 fame ratio : whence it is evident, that all cylinders of the 



13 j fame 



