D. CHEMISTRY AND METALLURGY. 209 



for the engraver and the hardness of steel for the printer. 

 It is true that the precipitated iron obtained by his process 

 is also very brittle, and, owing to the presence of hydrogen, 

 has a specific gravity of only 7.675; but the hydrogen can 

 be expelled by heating, and the iron left with a specific 

 gravity of 7.811, and perfectly malleable, very elastic, and 

 flexible like sheet-steel, and capable of being welded in 

 short, a perfect wrought iron. 5 (7, 1873, XL V., 359. 



THE GASEOUS, LIQUID, AND SOLID STATES OF WATER. 



Professor James Thomson, who has lately removed from 

 Belfast to the University of Glasgow, has, in connection with 

 his brother, Sir William Thomson, lately elaborated certain 

 theoretical views with regard to the nature of the vapor 

 given off by solid and liquid bodies, especially water, when 

 near the point of congelation. A body can exist in any one 

 of the three states gaseous, liquid, or solid ; when two of 

 these states are present in contact together, the pressure and 

 temperature are dependent each on the other; so that when 

 one is given, the other is fixed. If a curved line be drawn 

 such that the ordinate and abscissa at any point represents, 

 respectively, the tension and the temperature, there will be, 

 in general, three curves : one expressing the relation between 

 temperature and pressure for gas in contact with liquid, 

 another expressing that for gas in contact with solid, and 

 another expressing that for liquid in contact with solid. 

 These three curves must all meet, or cross each other, in one 

 point of pressure and temperature, which may be called the 

 triple point. The triple point is, in fact, what would often 

 be called the freezing-point in vacuo. Sir William Thomson 

 gives a formula by which the difference of the pressure of 

 steam in contact with water, and in contact with ice, for any 

 temperature very near the triple point, may be found, with a 

 very close approximation to the truth, he having made the 

 actual calculations for temperatures several degrees either 

 side of the freezing-point of water. He has made a careful 

 examination of the experimental results of Regnault, and 

 finds that the observations of that eminently careful investi- 

 gator give a clear indication of the truth and correctness of 

 the formula, according to which, in any small descent in tem- 

 perature from the triple point, where the pressure of steam 



