S3<£ ON PREVENTING THE DECAY OF WOOD. 



marked that on rubbing wood painted white, and long ex. 

 posed to the weather, the white lead has come off in a dry- 

 powder like whiting ; as if the vehicle which glued it to the 

 wood had been decomposed and lost, leaving only the pig- 

 ment behind: And I have been much iuclined to suspect, 

 that this has arisen from the oil having been too much 

 ope?ied y as the workmen call it, or having its thickness and 

 tenacity too much diminished by a superabundance of the 

 oil of turpentine. In this state it may, in various ways, 

 Acts similarly be more readily acted on by water and air. We know, that 

 in discharging ^ properties of what are called unctuous or fat oils are 

 much changed by the admixture of the volatile or essential 

 oils. On this principle we succeed in getting grease out of 

 woollen cloths by oil of turpentine ; bnt whether the same 

 change is produced on the drying oils, I have not learned. 

 Is the pigment It appears, then, that these drying oils either by them- 



Or usf* ' 



selves, or boiled with metallic oxides, will form a varnish 

 on wood ; but it may be questioned how far the colouring 

 matters, with which they are usually mixed, contribute to 

 increase their preservative power. I do not however, deny, 

 that they may be serviceable in this and other views. They 

 might be supposed to enable the oil to lay firmer hold, as it 

 were, on the wood; and they may serve to increase the 

 thickness of the defensive covering. The first of these 

 This doubtful, points is of some importance ; for we observe that the paint 

 on street doors, which is become thick by frequent incrus- 

 tation, is apt, from the strong influence of the summer's 

 sun, to separate from the polished wood beneath, and rise 

 in large blisters; probably in consequence of a greater ex- 

 pansion in the crust itself than in the subjacent wood. 

 Here, therefore, the colouring matter of the paint fails to 

 produce the desired effect; and as to the second end, or 

 that of increasing the thickness of the covering, that may, 

 probably, be much more effectually accomplished than by 

 the mere addition of pigments, some of which are capable 

 of chemical decomposition, and all are costly. This pur- 

 Road dust. P ose an ingenious artist has of late attempted to answer, by 

 recommending an admixture of road-dust; and for that and 

 other means of reducing the price of paints, has obtained 



3 premium 



