66 SECTIONAL ADDRESSES, 
and at 900° about 720mm., so that it increases rapidly with rise oi 
temperature. The dissociation pressure at the melting-point stated 
above was determined experimentally. It is quite probable that the 
high pressure of carbonic gas favours fluidity in the melt, and also 
accelerates crystallisation. 
If the pressure be less than the figures given above, a certain amount 
ot dissociation will take place, and lime, CaO, will be present in the 
melt. We have then a binary system CaO and CaCO, and a binary 
eutectic point is to be expected. Boeke has investigated this system, 
and finds that the eutectic mixture has a melting temperature approxi- 
mately 1218° C., and consists of 91 per cent. CaCO, and 9 per cent. 
CaO. Small additions of CaCO, raise the melting-point only slowly, 
but the presence of additional lime makes the melt far more in- 
fusible. | Mixtures of CaO and CaCO, with more than 9 per cent. 
CaO show on microscopic examination a finely crystalline first genera- 
tion of lime crystals followed by a second generation of CaCO, and 
CaO well crystallised. On the other hand, mixtures containing 
less CaO than the eutectic proportion show branching skeleton 
crystals of early CaCO, which have a development indicating trigonal 
symmetry, and a ground mass consisting of CaO and CaCO,. The large 
early skeleton crystals often continue to grow during the consolidation 
of the eutectic so as to give a coarsely crystalline appearance to the 
aggregate. Hence melts containing little lime often yield semi- 
transparent crystalline masses having the appearance of marble though 
not its minute structure. The refractive index of CaO obtained in this 
way is about 1.83, and it is optically isotropic, so that there is no 
difficulty in recognising it in the microscopic slides. 
So far as research has yet gone, no evidence has been found that 
there are intermediate compounds between CaO and CaCO,, and the 
two substances do not appear to form solid solutions to a perceptible 
extent. 
To indicate how far experimental methods have advanced since the 
days of Sir James Hall, a brief account of the apparatus used will not be 
without interest. The carbonate of lime was either true Iceland 
spar, which is quite as free from admixture as the best prepared 
carbonate, or specially purified precipitated CaCO,. It was heated in a 
platinum vessel, as in Hall’s experiments. This vessel was placed in a 
small electric resistance furnace with walls of fireclay and magnesia 
and a platinum spiral. This furnace could attain a temperature of 
1600° in a few minutes, and maintain it perfectly steadily for days if 
required. The small furnace containing the platinum tube was now 
placed in a steel vessel less than six inches in diameter with thick walls. 
The lid of the container was fastened with bolts and nuts and a lead 
washer used to prevent escape of gas. By this arrangement the small 
internal furnace was alone heated; the steel enclosing vessel could be 
kept cold if necessary by a water-cooling arrangement, and it was a 
fairly simple matter to obtain gas-tight connections, and to obviate any 
risk of bursting. The space between the steel vessel and the furnace 
was packed with purified asbestos, to prevent convection currents in the 
carbonic acid gas from affecting the temperature of the electric furnace. 
