434 EXPERIMENTAL INVESTIGATIONS. 



ist. To apply solid transparent cements which melt by 

 heat to object-glasses without danger of breaking or unanneal- 

 ing the glass. An iron plate, about one-quarter of an inch in 

 thickness, and at least an inch greater in diameter than the 

 glass, is placed on a ring of metal and well levelled ; on it 

 place a piece of soft filtering-paper, and on this the object- 

 glass, the flint glass being next the paper. Have the cement 

 melted in a porcelain vessel placed in a sand-bath, the tempe- 

 rature being just high enough to fully melt it but not to car- 

 bonise it, and keep it melted as short a time as practicable ; 

 then heat uniformly, by a spirit-lamp underneath the iron 

 plate, until the object-glass above is so hot that the hand can 

 just bear it ; remove the crown glass, pour the cement on to 

 the centre of the convexity in the flint and replace the crown 

 on it ; press with the finger on the centre of the crown until the 

 cement exudes all round from between the glasses ; remove 

 the lamp and support the plate of iron by a firm prop ; then 

 place a thick annular pad of filtering-paper, two-thirds of the 

 diameter of the glass, on the crown glass, and on this a mo- 

 derate weight, such as will press the glasses very close without 

 being so heavy as to distort them ; the weight had better be 

 as warm as the glasses. Props should be arranged so as to 

 keep the glasses in situ ; and if they move on each other, a 

 little adjustment may be made by the hands. Allow the 

 whole to cool slowly ; when cold, wash off the overflow of 

 cement with naphtha or other solvent, and the operation is 

 complete. 



2nd. To filter rosin or similar substances when not quite 

 clear. Procure the most transparent rosin ; I have obtained 

 some at drysalters' in Long Acre, which in films of O'l to 0*2 

 of an inch has scarcely perceptible colour. 



Make a single conical filter of white filtering-paper, sus- 

 pend it by the ring of a retort-holder placed near the top, 

 using no external funnel ; put the rosin, roughly broken, into 

 this filter, and place it a few inches in front of a bright par- 

 lour fire, turning it occasionally : the rosin will melt and drop 

 from the filter beautifully clear. It may be collected in a 

 vessel, or dropped at once into castor-oil or other liquid used ; 

 they should then be warmed and well stirred together, and 

 may be put aside for use when required. 



