VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 287 



above the iron was little injured. The fonding heat of the glass furnaces I 

 examined, or that by which the perfect vitrification of the materials is produced, 

 was at one of them 1 14° for flint-glass, and 124° for plate-glass; at another it 

 was only 70° for the former, which shows the inequality of heat, perhaps un- 

 known to the workmen themselves, made use of for the same purpose. After 

 complete vitrification, the heat is abated for some hours to 28 or 2Q°, which is 

 called the settling heat; and this heat is sufficient for keeping the glass in fusion. 

 The fire is afterwards increased, for working the glass, to what is called the 

 working heat; and this I found, in plate-glass, to be 57°. Delft ware is fired by 

 a heat of 40 or 41°; cream-coloured, or queen's ware, by 86°; and stone ware, 

 called by the French pots de gres, by 102°: by this strong heat, it is changed to 

 a true porcelain texture. The thermometer-pieces begin to acquire a porcelain 

 texture at about 110°. 



-The above degrees of heat were ascertained by thermometer-pieces fired along 

 with the ware in the respective kilns. But this thermometer affords means of 

 doing much more, and, going further in these measures than I could at first 

 even have expected; it will enable us to ascertain the heats by which many of the 

 porcelains and earthen wares of distant nations and different ages have been 

 fired: for as burnt clay, and compositions in which clay is a prevailing ingredient, 

 suffer no diminution of their bulk by being repassed through degrees of heat 

 which they have already undergone, but are diminished by any additional heat 

 (according to obs. 5), if a fragment of them be made to fit into any part of the 

 gage, and then fired along with a thermometer-piece till it begins to diminish, 

 the degree at which this happens points out the heat by which it had been fired 

 before. Of several pieces of ancient Roman and Etruscan wares, which I have 

 examined, none appear to have undergone a greater heat than 32°, and none 

 less than 20°; for they all began to diminish at those or the intermediate 

 degrees. 



By means of this thermometer some interesting properties of natural bodies 

 may likewise be discovered, or more accurately determined, and the genus of 

 the bodies ascertained. Jasper, for instance, is found to diminish in the fire, 

 like an artificial mixture of clay and siliceous matter; granite, on the contrary, 

 has its bulk enlarged by fire, while flint and quartzose stones are neither enlarged 

 nor diminished. These experiments were made in fires between 70 and 80° of 

 this thermometer. A sufficient number of facts like these, compared with 

 each other, and with the properties of such natural or artificial bodies as we wish 

 to find out the composition of, may lead to various discoveries, of which I have 

 already found some promising appearances ; but many more experiments are 

 wanting to enable me to speak with that certainty and precision on these subjects 

 which they appear to deserve. 



