VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 270 



in kind, and in many cases destroyed, by a small augmentation of the heat 

 which had produced them ; insomuch, that in the gradual increase of the fire, a 

 precise moment of time must be happily seized, in order to catch them in per- 

 fection : — and when inconveniences, similar to these, arise in operations by fire 

 on metals and other substances : — how much is it to be wished, that the authors 

 had been able to convey to us a measure of the heat made use of in their valu- 

 able processes ! 



In a long course of experiments, for the improvement of the manufacture I am 

 engaged in, some of my greatest difficulties and perplexities have arisen from not 

 being able to ascertain the heat to which the experiment-pieces had been exposed. 

 A red, bright red, and white heat, are indeterminate expressions ; and even 

 though the 3 stages were sufficiently distinct from each other, they are of too 

 great latitude; as the brightness or luminousness of fire increases, with its 

 force, through numerous gradations, which can neither be expressed in words, 

 nor discriminated by the eye. Having no other resource, I have been obliged 

 to content myself with such measures as my own kilns and the different parts of 

 them afforded. Thus the kiln in which our glazed ware is fired furnishes 3 

 measures, the bottom being of one heat, the middle of a greater, and the top 

 still greater : the kiln in which the biscuit ware is fired furnishes 3 or 4 others, 

 of higher degrees of heat ; and by these I have marked my registered experi- 

 ments. But though these measures had been fully adequate to my own views, 

 which they were not, it is plain, that they could not be communicated to others; 

 that their use is confined to a particular structure of furnaces, and mode of 

 firing ; and that, on any alteration in these, they would become useless and 

 unintelligible, even where now they are best known. And indeed, as this part 

 of the operation is performed by workmen of the lowest class, we cannot depend 

 on any great accuracy even in one and the same furnace. It has accordingly 

 often happened, that the pieces fired in the top of the kiln in one experiment, 

 have been made no hotter than those fired in the middle in another, and vice 

 versd. 



The force of fire, in its higher as well as lower stages, can no otherwise be 

 justly ascertained than by its effects on some known body. Its effect in changing 

 colours has already been hinted at ; and I have observed compositions of calces 

 of iron with clay to assume, from different degrees of fire, such a number of 

 distinct colours and shades, as promised to afford useful criteria of the respective 

 degrees. With this idea, I prepared a quantity of such a composition, and 

 formed it into circular pieces, about an inch in diameter, and a quarter of an 

 inch thick. A number of these were placed in a kiln, in which the fire was 

 gradually augmented, with as much uniformity and regularity as possible, for 

 near 6o hours ; the pieces taken out at equal intervals of time during the succes- 



