4 Transactions of the Society. 



compound. As the degree of concentration of salt in the original 

 solution increases, the initial freezing-point on the branch A B will 

 be lower and lower, while the second freezing-point always remains 

 constant at -22° C. or -8°F. ; and when the solution contains 

 23 * 5 per cent, of salt, both freezing-points coincide in the point B at 

 -22° Cor -8°F. 



A frozen solution of salt in water containing less salt than that 

 necessary to form the" eutectic, will therefore bo made up of ice sur- 

 rounded by eutectic, while one containing more salt than that necessary 

 to form the eutectic will be made up of crystals of salt surrounded by 

 eutectic. If one of these mixtures of ice and eutectic, or salt and 

 eutectic, be suitably prepared and examined under the Microscope it 

 will be found that, whilst the ice or salt crystals are homogeneous, 



Freezing-point Curves 

 of solution of Common Salt in Water. 



Per 

 Cent. 



1 



5 



1 



■a 

 t 



5. 



£ 



, (Common 

 30 Salt 



the cryohydrate or eutectic is made up of a mechanical mixture of 

 intimately mixed ice and salt laminae similar to that shown in plate I. 

 fig. 3, which is mainly an eutectic of iron and iron carbide. 



It will now be shown how closely structures obtained with frozen 

 masses of ice and salt are related to those obtained with alloys of iron 

 and iron carbide. 



It will hardly be possible on the present occasion to do more than 

 roughly consider the laws controlling the formation of the structures 

 of pure iron, steel, and white iron. The case of grey and mottled 

 irons is much more complicated, and will have to be left for the present. 



Fig. 2 is an equilibrium curve, by Sir W. Koberts -Austen, of 

 the carbon-iron series, and was obtained in a similar manner to the 

 ice-salt curve, although, of course, a pyrometer had to be substituted 



