RES 



RES 



fore, are generally considered as volatile 

 oils saturated with oxygen. The gene- 

 ral properties of resinous substances are 

 the following. They are solid, brittle, 

 and commonly of a yellowish colour, 

 with some degree of transparency. The 

 taste, resembling volatile oils, is hot and 

 acrid. They have no smell. The specific 

 gravity is from 1.01 to 1.22. All resinous 

 bodies are electrics, and when excited by 

 friction, the electricity is negative ; hence 

 it is called resinous electricity. They 

 melt by being exposed to heat, and burn 

 with a yellow flame, giving out a great 

 quantity of smoke. Resins are insoluble 

 in water. Resinous substances are solu- 

 ble in nitric acid ; part is precipitated by 

 the addition of water, and the whole by 

 means of the alkalies. With the assist- 

 ance of heat they are all soluble in alco- 

 hol, and in sulphuric ether. Resins are 

 soluble in some of the fixed oils, and also 

 in volatile oils. Resinous substances 

 have been found to be soluble in the fixed 

 alkalies. We shall enumerate some of 

 the resins which are best known, and 

 which have not already been described 

 in separate articles. 



Rosin. This substance is extracted 

 from different species of the fir, and the 

 resinous matter obtained from it has re- 

 ceived different names. That procured 

 from the pinus sylvestris is the common 

 turpentine ; from the pinus larix, Venice 

 turpentine ; and from the pinus balsamea, 

 balsam of Canada. The turpentine is ob- 

 tained by stripping the bark off the trees ; 

 a liquid juice flows out, which gradually 

 hardens. This juice consists of oil of 

 turpentine and rosin. By distilling the 

 turpentine the oil passes over, and the 

 rosin remains behind. By distilling to 

 dryness common rosin is obtained. When 

 water is added, while it is yet fluid, and in- 

 corporated by agitation, what is called 

 yellow rosin is formed. 



Pitch is a resinous juice obtained from 

 the pinus picea, pitch pine. It is purified 

 by melting and squeezing it through 

 linen bags, and it is then known by the 

 name of white, or Burgundy pitch. White 

 pitch mixed with lamp-black forms black 

 pitch. 



Sandarac. This resinous substance is 

 extracted from the juniper. It is a spon- 

 taneous exudation from this plant in the 

 form of brown tears, which are semitrans- 

 parent and brittle. See BALSAM, COPAL, 

 GUIACTTM, Sec. 



RESISTANCE, or RESISTING force, in 

 philosophy, denotes, in general, any 

 power which acts in an opposite direc- 



tion to another, so as to destroy or dimi- 

 nish its effect. Hence the force where- 

 with bodies, moving in fluid mediums, 

 are impeded or retarded, is the resistance 

 of those fluids. Authors have established 

 it as a certain rule, that, whilst the same 

 body moves in the same medium, it is al- 

 ways resisted in the duplicate proportion 

 of its velocity; that is, if the resisted 

 body move in one part of its track with 

 three times the velocity with which it 

 moved in some other part, then its re- 

 sistance to the greater velocity will be 

 'nine times the resistance to the lesser: 

 if the velocity in one place be four times 

 the velocity in another, the resistance to 

 the greater velocity will be sixteen times 

 the resistance to the lesser, and so on. 

 This rule, though very erroneous, when 

 taken in a general sense, is yet undoubt- 

 edly very near the truth, when confined 

 within certain limits. 



In order to conceive the resistance of 

 fluids to a body moving in them, Mr. Ro- 

 bins distinguishes between those fluids, 

 which being compressed by some incum- 

 bent weight, perpetually close up the 

 space deserted by the body in motion, 

 without permitting, for an instant, any 

 vacuity to remain behind it ; and those 

 fluids in which, they being 1 not sufficient- 

 ly compressed, the space left behind the 

 moving body remains for some time 

 empty. These differences in the resist- 

 ing 1 fluids will occasion very remarkable 

 varieties in the laws of their resistance, 

 and are absolutely necessary' to be con- 

 sidered in the determination of the action 

 of the air in shot and shells; for the air 

 partakes of both these affections, accord- 

 ing to the different velocities of the pro- 

 jected body. If a fluid were so con- 

 stituted, that all the particles composing 1 

 it wer.~ at some distance from each other, 

 and there was no action between them, 

 then the resistance of a body moving 

 therein would be easily computed from 

 the quantity of motion communicated to 

 these particles : for instance, if a cylinder 

 moved in such a fluid in the direction of 

 its axis, it would communicate to the 

 particles it met with a velocity equal to 

 its own, and in its own direction, sup- 

 posing that neither the cylinder nor the 

 parts of the fluid were elastic; whence, 

 if the velocity and diameter of the cylin- 

 der be known, and also the density of the 

 fluid, there would thence be determined 

 the quantity of motion communicated to 

 the fluid, which (action and re-action be- 

 ing equal) is the same with the quantity 

 lost by the cylinder ; consequently the 



