VOL. LXXVI.3 VHILOSOPHICAL TRANSACTIONS. 141 



T have mentioned also, in my former paper, that a quantity of air is extri 

 cated from the clay, most rapidly at the period in which the augmentation of 

 bulk, takes place ; and that the augmentation was probably owing to this air 

 forcing the particles of the clay a little asunder, previous to the instant of its 

 escape, ft was therefore presumed, that the greater extension of these new 

 clays might be owing, either to a greater quantity, or stronger adhesion, of this 

 combined air: and as clay, kept moist for a length of time, in certain circum- 

 stances, undergoes a process seemingly analogous to fermentation, it was hoped 

 that, by such a process, part of its combined air might be detached. But ex- 

 periments made on this idea have proved, that these clays, kept moist for a 

 twelvemonth, — kept for a considerable length of time in a heat just below visible 

 redness, — boiled in water for many hours, — alternately, and repeatedly, mois- 

 tened and dried, — suffer no alteration in their thermometric properties, and con- 

 tinue to differ from the standard clay just as much as they did at first. 



Some of these new clays differed from the old in a property still more essen- 

 tial, and by which I was much more disconcerted ; for though they continued 

 diminishing with tolerable regularity, keeping only some degrees behind it, up 

 to a certain period of heat, about that in M^hich cast iron melts ; yet many of 

 the pieces, urged with a heat known to be greater than that, were found not to 

 be diminished so much as those which had suffered only that lower heat. Fur- 

 ther experiments showed, that after diminishing to a certain point, they begin, 

 on an increase of the heat beyond that point, to swell again : and as this effect 

 is constant in certain clays, and begins earliest in those which are most vitres- 

 cible, and as clays are found to swell on the approach of vitrification, I consider 

 this second enlargement of bulk, however inconsiderable, as a sure indication 

 of the clay or composition having gone beyond the true porcelain state, and of a 

 disposition taking place tov/ards vitrification ; which stage is always, so far as my 

 experience reaches, attended with a new extrication of air ; and in some in- 

 stances, this air being unable to make its escape from the tenacious mass that 

 envelopes it, the burnt clay is thereby so much increased in bulk as to swim on 

 water like very light wood. The degree of heat therefore, at which this en- 

 largement begins, may be considered as a criterion of the degree of vitrescibility 

 of the composition ; which points out a new use of this thermometer, enabling 

 us to ascertain the degree of vitrescibility of bodies that cannot actually be 

 vitrified by any fires which our furnaces are capable of producing. 



All my researches among the natural clays proving fruitless, and the experi- 

 ments having shown that all those, which could sufficiently resist vitrification, 

 diminished too little in the fire, I endeavoured to find a body possessed of the 

 opposite property, that is, diminishing too much, and, by a mixture of these 

 two, to produce the medium diminution required. As I could not find any 



