Scott. — Resistance of Steel to Mechanical Shock. 521 



23 5 per cent, of salt was reached, when, as before, the salt and 

 ice crystals would separate out side by side, forming the in- 

 timate mixture, the cryohydrate or eutectic ; but in this case 

 we should have crystals of pure salt surrounded by crystals 

 of the eutectic. 



At each stage of the solidification there would be a retarda- 

 tion in the fall of the thermometer, which on reaching — 22° C. 

 would remain stationary till the whole mass solidified. It 

 is evident that for all mixtures other than 23 - 5 per cent, there 

 are at least two distinct retardations — the first, dependent 

 on the strength of the solution, occurring when the crystals of 

 ice or salt commence to separate out ; the second, when the 

 formation of the eutectic begins. Where the solution is ori- 

 ginally of eutectic proportions (23 - 5 per cent, of salt) only the 

 latter point of retardation occurs, there being no crystalliza- 

 tion until — 22° C. is reached. (The matter may be made clear 

 by reference to fig. 4.) 



SALT 



90 



to 



765 TO 

 S3. 5 3 O 



GO % 



4o/ 



Now, the behaviour on cooling of a liquid solution of carbon 

 in iron is very much that of the salt-in-water solution — if there 

 is less than about 4 - 5 per cent, of carbon present we have first 

 the separating- out of the iron, and then the solidification of the 

 eutectic of iron and carbon. If there is more than 4 - 5 per cent, 

 carbon present in the liquid solution the graphite first separates 

 out, then when the temperature has fallen sufficiently the eu- 

 tectic, the solution being by this time of euctetic proportion. 

 If originally of eutectic proportion (4-5 per cent, carbon) there 

 is only one point of retardation and solidification. But there 

 is this great difference between the ice and the iron : the latter, 

 on account of the high temperature at which it solidifies, re- 

 tains a certain amount of carbon after it becomes solid. Hence 

 there is a solid solution of carbon in iron which introduces fur- 



