[ 94 + ] 
to be an aflringent -mineral earth ; whereas charcoal 
eaiily takes fire,, burns freely without finoke, and 
continues burning, till it confumes to an afh ; which 
confiffs of an alkaline fait, and a pure earth, fit for 
making cuppels ; and, by thefe marks, is fufticiently 
diflinguifhed from all mineral fubifanees. 
Grofvenor-Street, Jan. 8, 1761. I er. Mille-s. 
LXXXVI. De Aberratione Luminis , in Su- 
per fcicbus £t JLentihus Sphezricis refra&o- 
rum *. 
% «• 
Read April 2, I radius luminis AG, incidens in fu- 
1 761 * perficiem quamcunque refringentem 
L E, infledatur 
fecundum rec- 
tam G K, et 
quffivis reda li- 
nea A K occurrat radio incident! A G in A, refrado 
G K in K j et redas G D, normali ad fuperficiem re- 
fringentem L E, in D j erit redangulum G K x D A 
ad redangulum GAxDK, ut finus anguli inciden- 
tias DGA ad finum anguli rcfradionis DGK. Elf 
enim DA ad G A, ut fin. DGA ad fin. ADG, et 
GK : DK :: fin. GDK : fin. DGK; quare, ob 
* This Paper, though Tent to England in the fummcr of the 
year 1760, was, by accident, prevented from being read to the 
Royal Society, till the 2d of April following. 
fin. 
