c 25i ON THE COLOUR SlC. OF THE METALLIC OXIDES. 



degree, the mixture spontaneously takes fire, and is suffered to 

 burn about a minute. The flame is then smothered, and the 

 residue pulverised, which forms a vlokl powder . This powder 

 being sublimed, affords a sublimate of a livid red colour, which 

 when powdered, exhibits a tine red colour, known by the 

 name of vermilion." — Here it is very obvious, that the high 

 degree of heat, necessary to produce this sublimate, dissipated 

 the violet colour, in consequence of its great refrangibility, and 

 fixed the red ray in the oxide, which constitutes the vermilio?i 

 colour. — To these I could add numberless facts, on the different 

 coloured oxides of the different metals, in support of the doc- 

 trine which I have adopted,. " but no more causes are necessary 

 than are sufficient to explain the phenomena." — Hence this 

 exposition most elegantly proves and illustrates the doctrine of 

 Sir Isaac Newton, on the seven different rays of light, and their 

 different refrangibilities and reflexibilities. 



It must now appear very evident, that a knowledge of these 

 principles, and an application of them to the arts, would in a 

 very great degree assist the manufacturers, and particularly those 

 who work in porcelain, china, glass, and in all kinds of pot- 

 tery, to burn in, and fix the different colours, according to their 

 different refrangibilities. — That is to say, the degree of heat 

 which would be necessary to fix permanently the red colour, 

 would be a temperature so high, as to burn out and dissipate, 

 all the other colours, provided all the seven coloured oxides, 

 were made from the same metal, and painted on a piece of 

 porcelain ; therefore to avoid an error of this kind, the manu- 

 facturer would be obliged to burn in the red colour first, second- 

 ly the orange, thirdly the yellow, fourthly the green, fifthly the 

 blue, sixthly the indigo, and seventhly and lastly, the violet co- 

 lour; for by an attempt to burn in and fix the violet colour first, 

 and afterwards to burn in the red, before the latter could be 

 accomplished, the former would be dissipated. — Therefore it 

 is necessary to know that the degree of heat sufficient to 

 produce the violet coloured oxide of gold, would be of so high 

 a temperature as to drive off all colour from the red oxide of 

 lead, and convert it into a white litharge : hence when several 

 colours are to be fixed in, or burnt on porcelain" at the same 



