342 THE WEDGWOODS. 



kind that are ground in them ; they are corroded and dissolved by 

 all acids, and hence, besides altering the nature of any acid liquor 

 put into them, by imparting to it as much of their substance as the 

 quantity of acid requires for its saturation, the surface of the marble 

 itself is rendered rough and cavernulous, and on that account still 

 more liable to be abraded, ancl very difficult to be made clean. Oils 

 of all kinds are imbibed by them, so that whatever follows an oily 

 substance in such a mortar must partake of the smell and taste of 

 the oil. METALLINE mortars are dissolved or corroded not only by 

 acids, but by all saline substances, by simple moisture, and by the 

 air ; and some experiments lately published by Mr. Blizard have 

 given grounds to apprehend that even dry substances of the mere 

 earthy kind, void of saline matter, and of no great hardness, will 

 receive, by being powdered in brass or bell-metal mortars, though 

 perfectly clean, a coppery impregnation, sufficient to manifest itself 

 in the common chemical trials, and perhaps not altogether innocent 

 in medicines or in aliments. From all these imperfections the 

 PORCELAIN mortars are free ; and their price is sufficiently moderate 

 to admit of their general use. This compact, hard porcelain is 

 excellently adapted also for evaporating pans, digesting vessels, 

 basons, filtering funnels, syphons, tubes such as Dr. Priestly uses 

 in some of his experiments instead of gun barrels retorts, and 

 many other vessels for chemical uses, which I have made for my 

 friends, of different forms and magnitudes, and with some variations 

 in the composition itself, according to the views for Which they 

 were wanted. If in this department I should be happy enough to 

 contribute anything towards facilitating chemical experiments, by 

 supplying vessels more serviceable or more commodious for par- 

 ticular uses than are commonly to be met with, my utmost wishes 

 with respect to these articles will be gratified." 



The last class (CLASS TWENTY) into which Josiah Wedg- 

 wood divided his productions was " thermometers for 

 measuring strong fire, or the degree of heat above ignition." 

 The principle on which these thermometers (accounts of 

 which had been, as I have stated, read before the Royal 

 Society) were constructed, was that of the shrinking of 

 earthly bodies of the argillaceous order by heat the diminu- 

 tion of their bulk being in proportion to the degrees of heat 

 to which they are subjected. The following is Wedgwood's 

 own account of this important invention : 



