148 CHYMISTRY APPLIED TO AGRICULTURE. 



are most numerous, as pines, cedars, &c. that it is princi- 

 pally extracted, and the term resinous is applied to them 

 from the very great proportion of resin contained in their 

 sap. 



The mode of collecting resin is by cutting notches 

 through the bark of the trunks of resinous trees near the 

 base, at that season when the sap, softened by the return- 

 ing warmth of spring, begins to rise in the vessels. As 

 resin abounds principally in the alburnum, the notches 

 must be of sufficient depth to pass through that ; the in- 

 cisions must be enlarged or renewed, once in fifteen days. 

 The flow of the resin ceases as the return of frost causes 

 the vessels of the trees to contract. A healthy and well- 

 grown tree will furnish from twelve to fifteen lbs. of resin 

 per annum. 



A different process is made use of in extracting resin 

 from dead trees; the bark and young branches of these 

 are taken off, and the remainder reduced to small pieces 

 which are piled up in a heap and covered over, excepting a 

 small opening which is left at the top : the heat of the fire 

 which is kindled at the upper part is sufiicient to melt the 

 resin which flows down to the bottom, and is carried off by 

 channels, into vessels prepared to receive it. 



This resin is black, and contains a great quantity of 

 pyroligneous acid and volatile oil : it is known in commerce 

 under the name of tar;* the quality of it varies according 

 to the care with which it is extracted: if the heat be too 

 great, the volatile oil is thrown off, and the resin rendered 

 dry and brittle: it cracks when used, and renders the 

 substances to which it is applied less ductile and pliable. 



The tar of southern climates has both faults ; and it was 

 formerly necessary that the marine arsenals should be sup- 

 plied with that made in the north of Europe ; but now that 

 tar is manufactured in furnaces, according to the process of 

 M. Darracq, in such a manner as to condense all the vola- 

 tile oil, and thereby render the tar more fat, unctuous, and 

 suitable for all purposes, it is employed for naval purposes, 

 equally with the best tar of the north. 



The resins are insoluble in water, but very soluble in 

 alcohol : they liquefy at a low temperature, and burn easi- 



* A description of the processes employed for extracting resins and 

 forming all the resinous preparations known in commerce, may be 

 found in my Chimie appliquie aux Arts. Vol. II. page 425-445, 



