MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
21 
are pressed, the smaller is the quantity of glass contained in them, and the 
less risk is there of the platina being broken when the finished glass is taken 
out ; but it is proper to avoid general contact between the corners and the 
sides against which they are disposed, otherwise welding is likely to occur 
during the stirring, and the platina is injured for future experiments. 
39. The tray being formed is again to be examined for holes, first by a light 
as before (31), and then in the following manner. Being laid upon a sheet 
of bibulous paper, alcohol is to be carefully poured in until the fluid is within 
the fourth or the sixth of an inch of the lowest edge of the tray, so as to 
occasion no running over at the sides or corners. If a large hole exist, it will 
be rendered visible immediately ; but if none such appear, a large basin 
or some other cover is to be placed over the tray to prevent evaporation, 
but without touching the vessel or its contents; and the whole is to remain 
undisturbed for some hours. Being then examined, the wetting of the paper 
will indicate a hole or a badly folded corner, and will point out the faulty 
place : the tray may easily be shifted from one part of the paper to another 
for the discovery of any moistened places beneath. Sometimes holes occur 
so small that alcohol will not run in a sensible quantity through them. 
Suspected places of this kind and suspicious corners also should be examined 
by a clean dry point of bibulous paper, which soon shows, by its change of 
appearance, the transmission of any fluid : but attention is required that no 
false indication be produced by carelessly bringing the paper near the upper 
edges of the platina, especially in the folded places. These minute holes do 
not occasion much harm in the furnace, but no fault should be allowed to 
pass which care can correct. 
40. When the tray is faulty, the alcohol must be removed by a small sy- 
phon, the holes soldered in the manner before described (32), and the tray 
again tried. When it proves good, it is, after the removal of the alcohol, to be 
heated red hot in every part by the flame of a large spirit lamp, and then 
reserved with care in a clean place until required. 
41. If the platina has been used before, it should first be ascertained that 
none of the glass from the former experiment remain on it. If there be any 
portion, the plate must be returned to the weak acid or pickle out of which 
it has been taken. If free from glass, it should then be examined as to any 
