298 Proceedings of the Royal Physical Society. 



it be of a granular nature, should form a thick stringy mass 

 when stirred, and the operator should then be able to judge 

 whether sufficient of the gum remains to cement the grains 

 together; if too much has been applied, more of the sub- 

 stance and a small quantity of pure ether must be added, 

 and the whole boiled over afresh. This mistake, however, 

 seldom happens after a little practice. When there is a 

 sufficiency of gum, the mixture should be kept boiling and 

 well stirred till it becomes of a reddish or brown colour ; 

 sometimes it is difficult to discern the colour, as the substance 

 interferes with it, but it can be seen in most cases. The 

 operator, however, can easily ascertain whether it has been 

 sufficiently boiled and has attained the necessary consistency, 

 by taking a little out on the point of a knife, and rapidly 

 cooling it by pressing against some cold surface, or holding 

 it a short time in water. If it hardens immediately it has 

 been boiled enough. 



The crucible can now be taken off the hot plate, and while 

 yet warm the substance should be scraped out with a knife, 

 and rolled or pressed with the fingers into an oblong mass ; 

 it is then ready for moulding, or it can be laid aside and 

 moulded at any time by gradually softening on a piece of 

 glass or in a porcelain dish upon the hot iron plate. The 

 moulds are easily made by cutting strips of ordinary tin four 

 inches long and three-quarters of an inch wide, bent tightly 

 over a round iron rod three-quarters of an inch in diameter, 

 a small slit being cut on one side of the tin to allow the 

 wire connected with the mould to sink below the surface of 

 the rim ; this permits the mould to stand level and close to 

 the glass plate; take the tin off the iron rod and bind it 

 firmly round with fine copper wire ; it is then ready for use 

 (PI. XL, Fig. 1). 



Moulding. 



This must be done while the substance is quite hot and 

 plastic. First put a piece of common flat glass three times 

 the size of the cavity of the mould upon the hot iron plate, 

 with the mould on the top and the notched end downwards, 

 so as to have it perfectly flat on the glass, and when quite 



