180 PHILOSOPHICAL TRANSACTIONS. [anNO 1765. 



red glass is produced. 2. When a small quantity of a solution of gold in aqua 

 regia is evaporated on a glass plate, with a gentle heat, that part of the glass on 

 which it lay thinnest, is tinged red, by the entrance of the particles of gold into 

 its surface. 3. Artificial rubies are made by mixing with glass, gold dissolved in 

 aqua regia, and afterwards calcined in the furnace. 4. Kunkel prepared a 

 powder for the same purpose, by precipitating the gold from the solution by an 

 alkaline liquor. 5. Gold precipitated by tin from aqua regia, and melted with 

 glass in a proper proportion, tinges it with a beautiful ruby colour : this method 

 was discovered by Cassius, and further improved by Kunkel. 6. The same 

 colour is produced by fusing gold with a large proportion of tin, and two thirds 

 of lead, or by mixing it with regulus of antimony, or tin by calcination, and 

 adding to glass the powders of gold obtained from these processes. 7. Gold 

 amalgamated with mercury, and digested with it for a considerable time, may 

 be reduced to a subtile powder by expelling the mercury: this powder, melted 

 into the glass, tinges it of a beautiful red. 8. Gold leaf melted into the sur- 

 face of glass, by the electric force, imparts a red colour to it : this was first ob- 

 served by Dr. Franklin, and has been often repeated. 



There are many other ways of communicating this colour to glass by gold ; 

 and I find no method by which it can be made to produce any other colour. If 

 it be mixed in larger masses, without being minutely divided, it imparts no 

 colour to the glass, but remains in its metallic form. Grummet attributes this 

 colour to the manganese used in making some sorts of glass, the colour of 

 which he supposes revivified by the nitre used in the preparation of the gold : it 

 is necessary therefore to mention, that I have given a red by gold to several 

 glasses, in the composition of which there was no manganese, and often by 

 gold in the preparation of which there was no nitre. Several preparations of 

 gold will impart a fine red to the frit or materials of which glass is made, in a 

 small degree of heat ; though not minutely enough divided, or in too large 

 quantity, to remain mixed with the glass, when exposed to a degree of heat suf- 

 ficient to vitrify them perfectly. 



LEAD, 

 which is the metal whose density is next in order to that of gold, affords a glass 

 of the colour of the hyacinth, a gem whose distinguishing character is, that it 

 is red with an admixture of yellow, the same colour which by writers on optics 

 is called orange. 



1 . Lead, kept in fusion for a considerable time, in a strong crucible and a 

 very violent heat, is reduced to a glass of the colour of that gem. 2. Lead re- 

 duced to litharge, and melted with one 3d or 4th part of its weight of sand, in 

 a covered crucible, in a strong fire for 2 or 3 hours, unites with the sand into 

 an orange-coloured glass iike the former. 3. Glass of lead is mentioned by 



