268' ' On the Tendency of 



I shall make no further comments on the 

 ^bovc experiments, by way of explanation ; 

 because to those who understand my hy- 

 pothesis of elastic fluids, they need none : and 

 I think: it would be in vain to attempt an ex- 

 piation any other way. I cannot however, on 

 this occasion, avoid adverting to some experi- 

 ments of Dr. Priestley, w^hich few modern 

 philosophers can be unacquainted with : I 

 mean those relating to the seeming conversion 

 of water into air. (Vid, Philos.Transact. vol. 73, 

 page 414 — or his Expts. abridged, vol. 2, 

 page 407.) He found that unglazed eartheu 

 retorts containing a little moisture, w^hen 

 heated, admitted the external air to . pass 

 through their pores at the same time that 

 aqueous vapour passed tbrough the pores the 

 contrary way or outward ; and that this- last 

 circumstance was necessary to the air's en- 

 trance. The retorts are air-tight, so far as that 

 blowing into them discovers no pores; but 

 when subjected to a greater pressure, as that 

 of the atmosphere, or even one much short of 

 it, they are not able to prevent the passage of 

 elastic fluids. The fact of air passing into the 

 retort through its pores, and vapour out of them 

 at the same time, are elegantly and most con- 

 vincingly §hewn by Dr. Priestley's experiments, 

 in which he used the apparatus represented in 

 plate 7, fig. 1, pf the edition above referred to. 



