On the Effeds produced on different Stones, &c. 29 
» to. Ts it well afcertained, as'M. Van Swinden thinks, that 
the aurora borealis ats on the magnetic needle; and can any 
idea be formed of the mode of this actioa? 
11. The fame queftion in regard to the zodiacal Hight 
12. In general, the theory of the magnet is {till fo foc froma 
perfection, even in that part which depeiids merely on-ob= 
fervation, that itis much to be wifhed that obfervers and 
~obfervations were multiplied, efpecially in what concerns 
\ 
the inclination of the needle... In:regard to: the declination 
and its variations, M. Van-Swinden has: given a noble ex+ 
ample of correctnefs, and of conftancy in obfervations, and 
in the art of claffing and compariag the refults: It would 
be of great benefit if this example were followed in different 
fituations and climates. It would be highly interefting, for 
infiance, to determine, with precifion, the zones of the earth 
where the declination is nothing, and where changes take 
place, and the fame for the cheek 
[ To be continued. } 
VII. On the Efizets produced on different Kinds of Stones by the 
Vapour of the Fluor Acid. By M. Kortum of Warfaw*. 
Pesine employed myfelf in engraving on glafs by 
means of the fluor acid, and having obferved that forae 
pieces of the glats I ufed were more fufceptible of being at- 
tacked by it than others, Lrefolved to try the effects of this 
acid on different kinds of vitreous flones. Conceiving that 
the fizures would be corroded with more neatnefs, the lefs the 
filiceous earth contained in the ftonss was mixed with fo- 
reign matter, I expofed.a very bright piece of rock-cryftal, 
from Swifferland, at a temperature of 18° Reaum. for twenty- 
four hours, to the vapour of the fluor acid; and found, con- 
trary to my expectation, that no impreflion was made upon 
it. I repeated the experiment, doub ling, the temperature 
and the time of expofure ; but the ‘fof did not Jofe any 
of its fpledor. It is well known that pure rock-cryftal 
* From /% oigt s Megazin fur den neuefen xufland der naturkunde, Vol. 1, 
Part 3. 
differs 
