20 
MR. FARADAY ON THE 
perpendicularly: this produces four projecting folded triangular comers, 
which being pressed close, are then turned against the sides, and a square tray 
is finished, which has no aperture or orifice below its upper edge. The fold- 
ing of these corners is a matter of much more consequence than might be 
anticipated. The plate is seldom so regular that the parts of two neighbouring 
sides which come together at a corner are exactly of equal height ; neither is it 
desirable that it should be so, and the unsymmetrical position of the guage to 
the plate, already recommended (35), is almost sure to prevent it. In that case, 
of the two sides of the folded corner, one will be higher than the other, and 
if the corner be so folded that its lower side is towards the tray and beneath its 
edge, a kind of syphon is formed which becomes charged with fluid by capil- 
lary action, and continues to discharge glass from the tray during the whole 
time of heating, notwithstanding that all the edges are much above the 
level of the fluid within. This in a long experiment is competent to occasion 
serious injury. 
3/. I have found, even when the edges of a corner have been of equal 
height, but below the edge of the side against which they are disposed, that 
still this capillary and syphon action has gone on, and the reason is not diffi- 
cult to comprehend ; the corners therefore have always been folded in such a 
manner, that their highest edge has been inwards, and both their edges above 
the level of the corresponding edge of the tray. To effect this, the line of 
their lateral flexure is not perpendicular to the bottom of the tray, but a little 
outwards above, and the proper degree of inclination is easily given by using a 
mould upon which to bend the corners. This should be a thick square piece 
of wood, having the four corners cut with different degrees of obliquity : when 
the corners of the tray are first imperfectly formed, it will be easy to ascertain 
by trial, which corner of this mould will give the 
obliquity and position already described as neces- 
sary, after which the folding may be easily finished 
upon it. The accompanying sketch represents first 
a good and then a bad folding. 
38. All occasion for changes in the folds, especially at the corners, should 
be avoided. The folds should be decided upon as the work proceeds, so 
advantageously as to make alterations unnecessary. The closer the corners 
