200 MR J. Y. BUCHANAN ON THE 



experimenter to receive the message, and if he succeeds he will have done a very fine 

 piece of work. 



§ 117. In considering the dilatation of the 7'225 CaCla solution before crystallisation, 

 we may pass over the first four series, although the oscillations of displacement 

 which they exhibit would be remarkable enough if they stood alone, and confine our 

 attention to the expansion in the fifth series which is continuous from the first to the 

 eleventh observation. The displacement at the first observation was 1245'92, and at 

 the eleventh 1246 "64, corresponding to an increase of 072 gram in ten minutes. But 

 the absolute minimum displacement observed was 1245*61 in the second series, so that 

 the extreme amplitude of expansion was 103 gram. While these changes of displace- 

 ment were going on, the liquid was perfectly homogeneous and its temperature was 

 absolutely constant. Therefore the dilatations were not of thermal but of mechanical 

 origin. We can apply no mechanical power which would produce such a stretching 

 eflfect on a liquid, but we can easily arrive at the mechanical power which could effectually 

 counteract it. 



Drecker * gives 0*0000217 as the coefficient of compressibility of a 40 '9 per cent, 

 solution of CaCl2, and we may take this as the compressibility of our 7*225 solution, 

 although it would be rather less. On this basis we obtain 38 atmospheres as the 

 pressure required to reduce the volume of the solution 7*225 CaCl2+ 1000 grams of water 

 from 1246*64 to 1245*61 cubic centimetres, and we conclude that, if we could place the 

 solution in conditions such that its internal pressure should be increased hy 38 atmo- 

 spheres, the extreme dilatation obsemed wo/dd he mechanically provided for. These 

 isothermal oscillations cease immediately when the first element of crystal appears in 

 the solution and aff"ords an outlet for its latent heat, after which crystallisation proceeds 

 in perfect tranquillity at a rate proportional to that at which heat is removed from the 

 solution. It is stopped if heat is supplied at this rate to the solution from without. 



§ 118. There is a remarkable resemblance between the state of unrest preceding 

 the crystallisation of a supersaturated solution and that preceding the liquefaction of 

 a gas, under a pressure not inferior to its critical pressure, when its temperature is 

 reduced slightly below its critical temperature. 



Andrews, in reporting his discovery of the critical state of liquids and gases, in- 

 cidentally describes this state of unrest as follows : — " On practically liquefying carbonic 

 acid by pressure alone, and gradually raising at the same time the temperature to 

 88° Fahr. (31*1° C), the surface of demarcation between the liquid and gas became 

 fainter, lost its curvature, and at last disappeared. The space was then occupied by a 

 homogeneous fluid which exhibited, when the pressure was suddenly diminished or the 

 temperature slightly lowered, a peculiar appearance of moving or flickering striae through- 

 out its entire mass. At temperatures above 88° Fahr. no apparent liquefaction of 

 CO2 or separation into two distinct forms of matter could be effected, even when a 

 pressure of 300 or 400 atmospheres was applied " {Phil. Trans. (1869) vol. clix. p. 575). 



* Wied. Ann., 1888, vol. xxxiv. p. 955. 



