239 



ner ; if, therefore, by a crack or scratch, a disjunc- 

 tion of the cohesion takes place,, in the internal sur- 

 face, the hidden action of the parts which remained 

 in a state of tension, to recover that of perfect co- 

 hesion, is supposed to effect the destruction of the 

 the mass. 



VARNISHING. 



By varnish is understood a clear limpid fluid, 

 capable of hardening, without losing its transpa- 

 rency ; used by painters, gilders, &c. to give a 

 lustre to their works, and to preserve and defend 

 them from the air and moisture. 



A coat of varnish ought to possess the following 

 properties: 1. It must exclude the action of the 

 air: because wood and metals are varnished to de- 

 fend them from decay and rust. 2. It must resist 

 water ; for otherwise the effect of the varnish could 

 not be permanent. 3. It ought not to alter such 

 colours as are intended to be preserved by this 

 means. It is necessary, therefore, that a varnish 

 should be easily extended or spread over the sur- 

 face, without leaving pores or cavities, that it should 

 not crack or scale, and that it should resist water. 



Resins are the only bodies that possess these pro- 

 perties, consequently they must form the basis of 

 every varnish. For this purpose they must be dis- 

 solved, as minutely divided as possible, and com- 

 bined in such a manner, that the imperfections of 

 those that might be disposed to scale, may be cor- 

 rected by others. 



Resins may be dissolved by three agents: 1. by 

 fixed or fat oil; 2. by volatile, or essential oil; 

 3. by spirit of wine. Accordingly, we have three 

 kinds of varnish : fat, or oily varnish ; essential oil 

 varnish ; and spirit varnish. 



