OK COPAL VARNISHES. 153 



oally ferved by the detedion of errors, as by the promulgation 

 of new fa6^s, I Iruft you will favour me by fuferting the follow- 

 ing obfervations on thofe pafTages which I have quoted from 

 Mr. Tingry's work; beginning with that in which he attempts 

 to demohlh one part of difcovery (if it may be fo called) with* 

 out ufing one experiment as the inftrument of deftru6tion. 



*' In regard to the means propofed by the medium of am- 

 :inonia, the faline nature of that liquid, if theprocefs fucceeds, 

 'Will not admit of the produ^ being placed in i//e class of var- 

 jNishes, deftined for delicate painting; it is a kind oi' fapona- 

 • ceous compound, the ufe of which is not to be recommended injuch 

 -cafes" 



In oppofition to all this I have but a (imple fad to produce: Inftances of 

 viz. It is many years fince I diffolved copal by the procefs al-g^ withthecopi 

 luded to; and adually varnithed feveral pi6tures with the folu- many years ago, 

 tion fo prepared: thofe pidures appeared to fliine, or bear out, J"^ ^^^^cd-' 

 !as artifts call it, as well as if they had been covered with any lent condition* 

 other varnifti; the colour of the varnitli has not changed in the 

 leait, which raoft other vamifhes would have done in the fame 

 time; they have ftood in a fraoky houfe in London the whole 

 time, have been repeatedly waflied with warm water, and 

 cleaned withfpiritof turpentine; and notwithftandingall this, 

 ihey ftill preferve their glolTy appearance undiminiflied. If a 

 folution which is proved by the experience of many years tp 

 potfefs thefe properties in an eminent degree, does not deferve 

 to be admitted into the clafs of varnithes, it is not eafy to tell 

 to what fubftance Mr. Tingry will grant that diftindion. 



Having faid thus much for the faft, we may venture to pro- The ammonia 

 ceed to the theory : When ammonia and copal were mentioned, P'o^^^^y fo^nis 

 the combination of both thofe fubftances with the fpirit of tur- compound; but 

 pentine appeared to Mr. Tingry to be fo much a matter of '^ss of no 

 courfe, that he took it for granted, and drew all the conclu- q^gncg^ 

 lions from it without making a (ingle experiment to prove the 

 reality of the fa6l, had he done fo, even he, with the ftrong 

 prejudices he evidently has upon this fubjed, might have 

 doubted if ths ammonia entered into the compofitinn at all; to me 

 it appears, after attending carefully to the phenomena of the 

 folution, that it does not', but without determining pofitiveiy 

 on the fubjed, I (hall defcribe thofe phenomena, and leave 

 your readers to judge for themfelves. 



The 



