On the Huttonian Theory of the Earth. 1^5 



that operation was perfect: or confufed, it denotes at leaft an union of particles previoufly 

 difperfed through a liquid medium, they muft, therefore, be at liberty to move through 

 this medium in order to coalefce, and re-unite to each other (if both they and the medium 

 itfelf coalefce and confolidate), this action is called coagulation, as happens in what was 

 called offa Helmontii, and the jelly formed by the liquor ftlicum ; but in Sir James's ex- 

 periment we find the confolidation to take place in a fragment of glafs, which ftill retained 

 its folid Hate, and, confequently, the particles were not at liberty to move towards each 

 other. This confolidation muft, therefore, evidently have arifen from fome internal 

 change in tht, conftitution of the glafles in which it was obferved. What thefc changes 

 may have been I fhail now examine. 



In the firft place, it is highly probable that filex, argil, and lime, and (lightly oxygenated 

 calx of iron, whatever be their affinity to each other when duly proportioned, require, 

 like all folids, to abforb in their paflage to a liquid ftate a certain portion of latent heat ; 

 but when in fufion, and the particles of each chemically united, they require a higher de- 

 gree of heat to keep them in fufion ; their eledlive affinities promoting fufion before the 

 union, and impeding it after the tmion is formed; it is thus that iron and platina, metals 

 feparately highly infufible, contribute to each others fufion; but when fufed, become ftill 

 more infufible, as appears by Rinm. §. 135. Sulphur and lead are feparately and eafily 

 fufible, but when united, their fufion becomes much more difficult. Again, Dr. Kennedy 

 has difcovered that all thefe whins contain 10 per cent, foda ; and Vauquelin has lately 

 difcovered tartarin in felfpar : in the high heats to which thefe ftones are expofed in order 

 to vitrify them, may it not'be fuppofed that thefe falts are, in fome meafure, volatized, and 

 the compounds thus rendered lefe fufible ? Though in an high-heat rapidly produced, they 

 may ftill be fufible as a fmaller proportion of foda will in that circumftance fuffice to 

 that effedt. 



The next circumftance to be accounted for, is the faxification or Jlony appearance 

 aflumed by the vitrified ft»nes when flowly cooled, by far the moft curious fa6t, for which 

 we are indebted to the ingenuity of Sir James. To account for this change, it is proper to 

 remark,- that though whins arc faid to be vitrified in a high degree of heat, yet this is not 

 rigoroufly true, for in that cafe theyftiould afford a tranfparent glafs, whofe fra<Sl:ure would 

 be perfedtly poliflied with a ftrong luftre, as we fee that of common glafs, whereas, in 

 truth, they melt only into an enamel, nearly approaching to the perfedl vitreous ftate : even 

 the bottles made of them are nothing more ; and hence their fuperior hardnefs. Their 

 ingredients, therefore, are not uniformly diffufed through their whole mafs, but lie in the 

 fame order and pofition as before fufion, and in eiFe£l they contain much more filex than 

 can be compleatly vitrified by the fmall proportion of lime and argill that enter into their 

 compofition, even though affifted by the foda ; and in the next place we muft notice, that 

 the affinity of foda to filex diminiffies in the fame ratio as the heat diminifhes, and, con- 

 fequently, they feparate, if the heat be not fo fuddenly diminiftied as to impede all motion. 

 This is evident by what happens to common glafs when flowly cooled down to the tem- 



S 2 perature 



