to detet'mining the jnagjiitude of material particles. 15 



I do not think it could be conducted to the requisite point of 

 closeness without an opposite result taking place to the open- 

 ing of the pores of Staffordshire ware by intense heat, namely, 

 a closing up of all pores by a vitrification of the body; the 

 Chinese ware being evidently very siliceous, and corre- 

 sponding in its transitions with mixtures I have already re- 

 ferred to. 



There is a brown stoneware manufactured in a large pot- 

 tery at Belper in Derbyshire, which, from certain qualities, 

 I should judge to be formed of a body likely to yield, at dif- 

 ferent degrees of heat, the several approximations of parts 

 suited for these experiments, provided it were protected from 

 a glazing of the surface, which would cause an imperfect re- 

 sult, tliough no glazing is impermeable by fluids under press- 

 ure. And it is proper to remark, that although I have de- 

 scribed only four degrees of density, they were selected out 

 of several yielding intermediate results ; which would be re- 

 quisite before the heat for the experiments were obtained. 

 These remarks may not appear irrelevant, since they may 

 serve as some guide to a person desirous of repeating or ex- 

 tending the experiments. 



The familiar fact that water may be retained in a porous 

 earthen pitcher for a length of time without any escaping ex- 

 cepting by evaporation, although the lower part is under the 

 pressure of a foot or more of the fluid, while such a pressure 

 would suffice to force air quickly through the pores, would 

 commonly be explained by supposing a greater minuteness of 

 parts in the case of air than of water; but the above experi- 

 ments would prove the contrary. 



The right explanation is to be found in the case of the li- 

 quid, in the cohesive attraction between the particles which 

 indispose the liquid to that breaking up into minute portions 

 requisite before it can enter the pores, and where it has en- 

 tered the pores in the capillary adhesion to the substance 

 of the ware by which the liquid refuses to leave its outer 

 surface and run down ; while in the case of the gaseous fluid 

 a mutual repulsion favours the separation of its particles, and 

 there is no considerable attraction between them and the earthy 

 surfaces to resist their passage out ; the greater subtilty of 

 the air over the water being due to these causes and not to its 

 being composed of minuter particles. In some measure re- 

 lated to this subject I had to notice some interesting facts 

 connected with the transmission of saline liquids through 

 woody fibre, but the space of this paper does not admit of their 

 being detailed here. 



Julius Jeffreys, 



