Translation o/"Rey's Essays. 137 



citation, No. 101, section 18, he supposes that the increased 

 weight of calcined lead is occasioned by its aerial parts 

 being consumed by the fire ; for which same reason, he says, 

 " a baked tile weighs more than an unbaked one." Oh ! 

 how resemblances often deceive ingenious minds ! This great 

 man, seeing the calx of lead and the tile become heavier, 

 after passing through the fire, supposing the effect the same, 

 sought only one same cause. It is however very different. 

 The tile increases in weight by dimunition of bulk, the calx by 

 the matter united with it. To make this more intelligible, who 

 is ignorant that the tile is formed of a fat and sandy earth, 

 kneaded with water, and that the sun, absorbing its moisture, 

 leaves in it an infinite number of little caverns which the water 

 had previously filled ? When baked in the furnace, the heat 

 softens it, as it softens metals, and almost brings it into fusion, 

 and in fact does fuse it, if the heat be excessive. In this 

 softening, the parts contract, unite and stick together; the 

 cavities disappear, and diminution of bulk ensues; whence its 

 greater weight, as I have often said before. As for the lead, it 

 fuses in the fire, as is known, and being melted, touches the 

 vessel in all parts, not leaving the smallest portion of air 

 within itself, according to the privilege which nature has be- 

 stowed on ponderous and fluid substances, to force upwards those 

 that are less so, always sinking down in them. This lead, set 

 aside, cools by degrees ; whilst its parts re-unite, and it con- 

 geals, sinking down in itself, and diminishing in volume, as 

 appears by the little hollow, observed on the top when it 

 is cold : so that we cannot suppose any air to be enclosed 

 in this heavy mass. Re-melt and calcine it, and you will find 

 it heavier, not from the consumption of aerial parts, for it had 

 none, but on account of the denser air united to it, as I 

 have already said. And in point of fact, if it lost any aerial 

 parts, would it not diminish in volume.^ On the contrary, its 

 bulk increases. Besides, if this opinion were correct, why do 

 not stones and plants gain weight by calcination ? Hence I 

 infer that the consumption of the aerial parts never increases 

 the weight of substances, wlien not followed by a diminution of 



