AO 
-by fuddenly heating a piece of Glafs in one place more then in another: 
which they are contiguous. 
‘by two kindsof Inftruments, either bya glowing hot round Iron*Ring; — 
| which tee incompafies the’ placethat is to be cut ; orelfe by a Sulphur d 
‘Three | 
-firft crack fallinto pieces,whereas if the Plate has becn gradually heated 
MrcroOGRAPHITA. 
I fuppofe proceeds partly from the Waters unequally cooling and pref: 
fing the partsof the drop, and partly from the felt-contra¢ting or fubfi- 
ding quality of the fubftance of the Glafs: For the vehemency of the 
heat of the drop caufes fuch fudden motions and bubbles in the cold Wa- 
ter,that fome parts of the Water bear more forciblyagain{t one wn then. 
againft another , and confequently do more fuddenly cool thole parts to 
‘A Second Argument may be drawn from the Experiment of cutting 
Glafles with a hot Iron. For in that Experiment’ the top of the Iron 
heats, and thereby rarifies the parts of the Glafs that lie juit before the 
crack , ‘whence each ‘of thofe agitated parts indeavouring to expand its 
felf and get elbow-room, thrufts off all the reft.of the contiguous parts, 
and confequently promotes the crack that was before begun. ~~ " 
A Third Argument may be drawn fromthe way of producing crack 
ina found piece or plate of Glafs, which is donetwo wayes, either Firft, 
And by this means Chymifts ufually cut off the necks of Glafs-bodies, 
ywhich is often wound about the place where the feparation isto 
be made;and then fired: ‘Or Secondly; A Glafs may be cracked by cooling _ 
it fudden! in any place with Water, orthe like, after ithasbeenalblei= 
furely and gradually heated very hot. Both which Phewomena feem ma- 
nifeftly to proceed fromthe expanfion and contraction of ‘the parts of 
the Glafs , which is alfo made more probable by'this circumftance which 
I have obférved , that a piece of common window-glafs being heated in 
the middle very fuddenly with a live Coal or hot Iron,does ufually atthe | 
very hot, and a drop of cold’ Water and the ‘like be: pution the mid 
a of it, it only flaws it, but does not break it afundér® immedi- 
ately, x 1Gled en bonrag 
. AF ourth Argument may be drawn from this’ Experiments? Take a 
Glafs-pipe, -and fit into ita folid ftick of Glals, 0 a8it will but jutt be mo- 
ved in iti» Then by degrees heat them whilft. they are oneywithin ano- 
ther,and they will grow ftiffer, but when they. are again cold;they will be. 
as eafie to be turned as before. © This Expanfion of Glafé isimore mani- 
feft in this: Experiment: 2) > IGqs 1990 Y1OV 905 doidy AWOMRTE 8 
Takea ftick of Glaf of a confiderable length and fir it fo betweenthe - 
two ends or ferews of a Lathjthat it'may bue juft éafily turngand thatthe — 
very ends of it may be jufttoucht'and fuftcined thereby 3 themapplying — 
the flame of the Candle tothe middle of it, andsheating it hot, you will 
prefently find the Glals to ftick very faft on thole pbints; ahd hot without 
much difficulty to be convertible onthem; | beforethar by ré g the 
flame fora 'while from it, it be fufferedito cool}anden . younwill findat 
aseafie to be turned round asat thefirft. © 3: motiodti to dagnolelade | 
Fromal] which Experiments it is-very evident } that albthofe Bodies, 
and particularly Gilat futlers an’ Expanfion by Paes ac a 
: - fiderable 
