ON PREVENTING THE DECAY OF "WOOD. 331 



a?r can be prevented, the wood is so far secure against de- therefore to b» 



mi* . • . i. -ii * i u • excluded, 



cay. a his principle may be illustrated by supposing a cy- 

 linder of dry wood to be placed in a glass tube or case, 

 which it exactly fills, and the two ends of which are, as it 

 is called, hermetically sealed, that is, entirely closed by 

 uniting the melted sides of each end of the tube. Who 

 will doubt that such a piece of wood might remain in the 

 open air a thousand years unchanged? Or let us take a still Thus amber 

 more apposite illustration of this fact; that of amber, a^ts^&c. 11 *" 

 native bitumen, or resin, in which a variety of small flies, 

 filaments of vegetables, aud others of the most fragile sub- 

 stances are seen imbedded, having been preserved from de- 

 cay much longer probably than a thousand years, and with 

 no apparent tendency to change for ten times that period. 

 Let us see then if we cannot, by the exclusion of moisture 

 and air, find means of virtually placing our timber in a case 

 of glass or amber. 



With this view, various expedients have been employed, Paint employ* 

 of which the most common is covering the surface with yiew ' 

 paint ; which is oil mixed with some substance capable of 

 giving it the colour which we desire. It is well known, that 

 several of the oils, as those of linseed, hempsced, &c, be- 

 come dry when thinly spread on any hard substance. The 

 drying quality is much assisted by their being previously 

 boiled with certain metallic oxides, more especially that of 

 lead, litharge. The crust so formed is with difficulty pe- 

 netrated by moisture or air. For this purpose drying oil is 

 Spread on silk or linen, in the manufacture of umbrellas; 

 and will tolerably well succeed in confining hidrogen gas, 

 or inflammable air, in the construction of air-balloons. 

 Hence we see the mode in which the application of paint on 

 wood serves to defend it against the causes of destruction. 



When paint is employed within doors, it is customary to Uses of oil of 

 add to the oil, beside the colouring matter, some essential . t t ur I )entme m 

 oil of turpentine, which not only makes it dry more rea- 

 dily, but, by giving it greater tenuity, causes it to flow 

 more freely from the brush, and therefore to go farther in 

 the work. For the same purposes I observe it forms a part 

 of the paint used on wood and iron work in the open air; 

 Jjut, as it appears to nie, roost improperly : For I have re- its disadvan- 



marked^**- 



