t6l Mf. Delaval on the Cauje of the 



When Pellucid Colourlcfs Glafs is broken, it 

 fometinnes happens that, the frefh furface of the 

 fragment is very fmooth and glofTy, If fuch a 

 fragnnent of glafs be fo placed, that the frelh 

 broken part may be feen at the farther furface, 

 that part cannot be diftinguiftied by the fight, 

 from a fimilar portion of glafs covered with 

 quickfilver. 



This efFe6t is produced, by the copious reflec- 

 tion of light from the broken part, which greatly 

 exceeds in fmoothnefs, the original furface of the 

 glafs. For, melted glafs, from its tenacious con- 

 fifl:ence, afTumes, whilft it cools, a furface lefs 

 glofly than thofe which are expofed, by breaking 

 it. Many other like inftances may be adduced. 



Thefe phsenomena afford much infight into the 

 nature, and caufe, of opacity. As they clearly 

 Ihew, that even the rareft Tranfparent Colourlefs 

 Subftances, when their furfaces are adjacent tQ 

 media greatly differing from them in refradive 

 power, may thereby acquire a perfecSt opacity, and 

 may aflume a refplendency, and hue, fo fimilar to 

 that of white metals, that the rarer pellucid fub- 

 ftances cannot, by the fight, be diftinguiflied from 

 the denfe opake metals. And, this fimilarity to 

 the furfaces of metals occurs, in the rare pellucid 

 fubftances, not only when, from the roughnefs of 

 their furfaces, they refemble unpoliflied metals in 

 their yivid whitenefs ; but alfo when, from their 



fmoothnefs. 



