132 



SIR H. DAVY 3 AGRICULTURAL CHEMISTRY. 



almost loo hot to hold. I have found that 

 a 2 inch bolt, 12 feet long, and 2 inches 

 diameter was torn asunder horizontally 

 by a machine, which is on the lever prin- 

 ciple, like the wcigh-bridges in the Royal 

 Dock Yards, with a strain of 82 tons ; 

 it began to stretch with 47 tons, and 

 lenghtening during the experiment 2 feet 

 9 inches, and was reduced at the point 

 of rupture to 1 1^ inches diameter. — Capt. 

 Broivn's Report on the "proposed Si. 

 Catharine's Bridge. — ib. 



SIR H. DAVY S AGRICULTURAL CHEMISTRY. 

 (Continued from \). 126.) 



13. Resin is very common in the vege- 

 table kingdom. One of the most usual 

 species is that afforded by the different 

 kinds of fir. When a portion of the bark 

 is removed from a firtree in the spring, 

 a matter exudes, which is called turpen- 

 tine; by heating this turpentine gently, 

 a volatile oil rises from it, and a more 

 fixed substance remains; this substatice is 

 resin. — The resin of the fir is the sub- 

 stance commonly known by the name of 

 rosin; its properties are well known. 

 Its specific gravity is 1072. It melts 

 readily, burns with a yellow light, throw- 

 ing off much smoke. 



Resin is insoluble in water either hot 

 or cold ; but very soluble in alcohol. 

 When a solution of resin in alcohol is 

 mixed with water, the solution becomes 

 milky ; the resin is deposited by the 

 stronger attraction of the water for the 

 alcohol. 



Resins are obtained from many other 

 species of trees. Mastich, from the Fis- 

 iacia lentiscus; elemi, from the Amyris 

 eletnifera; copal, from the Rhus copal- 

 linum; sandarach, from the common 

 juniper. Of these resins copal is the most 

 peculiar. It is the most difficultly dis- 

 solved in alcohol; and for this purpose 

 must be exposed to that substance in 

 vapor, or the alcohol employed must hold 

 camphor in solution. According to Gay 

 Lussac and Thenard, 100 partsof common 

 resin contain : 



Carbon 75.944 



Oxygen 13..337 



Hydrogen - - - - 10.719 

 Or of carbon - - - - 75.944 

 Oxygen and hydrogen in the 



proportions necessary to form 



water 15.156 



Hydrogen in excess - - S.900 



According to the same chemists. 100 

 parts of copal consists of 



Carbon 76.811 



Oxygen 10.606 



Hydrogen - _ _ _ 12.585 

 Or carbon - . _ - 76.811 

 Water or its elements - - 12.053 

 Hydrogen - - - - 11.137 



From these results, if resin be a definite 

 compound, it may be supposed to consist 

 of 8 proportions of carbon, 12 of hydro- 

 gen, and 1 of oxygen. 



Resins are used for a variety of pur- 

 poses. Tar and pitch principally consist 

 of resin, in a partially decomposed state. 

 Tar is made by the slow combustion of 

 the fir; and pitch by the evaporation of 

 the more volatile parts of tar. 



Resins are employed as varnishes, and 

 for these purposes are dissolved in alcohol 

 or oils. Copal forms one of the finest 

 It may be made by boiling it in powder 

 with oil of rosemary, and then adding 

 alcohol to the solution. 



14. Camphor is procured by distilling 

 the wood of the camphor tree {Laurus 

 camphora.) which grows in Japan. It 

 is a very volatile body, and may be puri- 

 fied by distillation. Camphor is a white, 

 brittle, semitransparent substance, having 

 a peculiar odor, and a strong acrid taste- 

 It is very slightly soluble in water, more 

 than 100,000 parts of water are required 

 to dissolve 1 part of camphor. It is very 

 soluble in alcohol; and by adding water 

 in small quantities at a time to the solu- 

 tion of camphor in alcohol, the camphor 

 separates in a crystallized form. It is 

 soluble in nitric acid, and is separated 

 from it by water. 



Camphor is very inflammable; it burns 

 with a bright flame, and throws off a great 

 quantity of carbonaceous matter. It forms, 

 in combustion, water, carbonic acid, and 

 a peculiar acid called camphoric acid. 



No accurate analysis has been made of 

 camphor, but it seems to approach to the 

 resins in its composition, and consists of 

 carbon, hydrogen, and oxygen. 



Camphor exists in other plants besides 

 the Laiirtis camphora. It is procured 

 from species %f the laurus growing in 



