Mr. S. P. Thompson on Permanent Plateau's Films. 271 



ascertain the best proportions. When the mixture contained • 

 a less proportion of balsam than 35 per cent, the films were 

 too brittle, and irregular in form. If it contained more than 

 70 per cent, of balsam the films did not readily harden, and 

 were not formed without difficulty. A mixture of 55 per cent, 

 of resin with 45 of balsam, which fused about 85°, gave good 

 films, tough on cooling, but somewhat brittle. The mixture 

 yielding the most satisfactory results contained 46 per cent, of 

 resin and 54 of balsam. This mixture is sufficiently fused at 

 80° to be workable, but yields the best films at 93° to 95°. 

 At 105° films can be obtained; and they are thinner than those 

 formed from the more viscid fluid at 95°. At 110° films are 

 still obtainable; and they frequently exhibit chromatic pheno- 

 mena, but usually burst before hardening. 



[The specimens exhibited to the Society are made with this 

 mixture. They include a cubical frame of 2*5 centims. side, 

 and a tetrahedral frame of 3'1 centims. side. Larger specimens 

 have been obtained, however, though they generally show 

 some imperfection of form, I have had a flat circular frame 

 of 11 centims. diameter covered with a film of beautiful trans- 

 parency. Brass wire appears better than iron for the frames.] 



The films made with the mixture described are remarkably 

 tough, and if preserved from rough handling appear to be of 

 indefinite durability. A number of frames holding films have 

 been hanging for over two months unprotected upon the wall 

 of the laboratory of the writer, and are still intact, Brass 

 wire of 0'33 millim. in diameter has been employed for the 

 construction of the frames. When a thicker wire is used, the 

 films become irregular from the longer retention of heat by 

 the wire, and the consequent earlier cooling of the central 

 portions of the films. 



As with the soap-films, so with those of resinous matter, 

 success depends largely upon the purity of material employed. 

 Dust and oily matters must be scrupulously excluded ; and 

 the resin should be retained at a temperature near its boiling- 

 point for some time, to purify it of more volatile matter, before 

 the balsam is mixed with it. 



The most perfect films are obtained when the wire frames, 

 after being dipped in the liquid, are removed to an air-bath 

 at the temperature of about 80°, in which they are left, and 

 the whole is allowed slowly to cool. 



In proof of the toughness of the films, it may be mentioned 

 that a recent flat film upon a circular frame of 4 centim. dia- 

 meter of iron wire of 0*9 millim. gauge sustained, without 

 breaking, the pressure of a cylindrical fifty-gramme weight, of 

 24 millims. diameter, placed upon its centre. 



