SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 197 



it may be, is an operation which has some of the elements of an act of creation — some- 

 thing appears where there was nothing of the kind before. The actions and reactions 

 which take place before the first element of crystal appears are withdrawn from our 

 sight, but their existence has been revealed by the hydrometer, which faithfully reports 

 to him who can use it the dilatations and contractions which precede the crystal's birth. 



They are illustrated in the accompanying diagram. In it the ordinates represent 

 displacements of the solution and the abscissae intervals of time, dating from the first 

 observation of the first series. The lowermost curve. No. 1, represents graphically the 

 data of displacement relating to the supersaturated solution 7*225 CaCla+lOOO grams 

 of water given in Table IIa. The uppermost curve, No. 2, represents those relating to 

 the non-saturated solution 6*3 CaCl2+ 1000 grams of water given in Table IIb. ; and the 

 intermediate curve. No. 3, represents the data relating to the supersaturated solution 

 7"196 CaCla+lOOO grams of water given in Table lie. 



The displacements are plotted in the order in which they were observed, and the 

 series follow each other in chronological order. The letters A, B designate the hydro- 

 meter which was used for the series represented under each respectively. The time 

 intervals between successive series of observations are included in the diagram and are 

 traced by dotted lines. The interval of time which separated two consecutive series 

 of observations was on an average seventeen minutes, and the duration of each series of 

 experiments was about fifteen minutes. The diagram shows at a glance the contrast 

 between the tranquillity of the 6 '3 CaClg solution and the unrest indicated by the 

 curves for the two supersaturated solutions. 



The curve No. 2 for the 6*3 CaClg solution pursues an even course, the displacements 

 oscillating between the extremes 1205'00 and 1204'89. Otherwise the curve differs 

 little from a straight line, and there is perfect agreement between the last result in one 

 series and the first in the succeeding series. This is shown by the horizontality of the 

 four dotted lines connecting the successive serial curves. 



Curve No. 1 for the 7*225 CaClg supersaturated solution is in striking contrast with 

 No. 2. There is little agreement between the displacements in any of the corresponding 

 series, and the oscillations of the serial curves are very marked, culminating in the 

 continuous expansion shown in series 5, after which crystallisation took place. This is 

 also well shown by the difference between the last displacement of one series and the 

 first displacement of the following series. It is evident that the state of unrest con- 

 tinued when the solution was left to itself in the cylinder. The slight contraction shown 

 in passing from the third to the fourth series indicates an effort on the part of the 

 solution to regain a more stable condition. It is, however, clear that the state of unrest 

 continued during the whole of the 140 minutes represented by the line of abscissae in 

 the diagram, and it suggests the possibility, and indeed the probability, that the super- 

 saturated solution, even when confined in a closed vessel, may never be at rest. 



§ 116. If we consider attentively what took place before the supersaturated solution 

 of calcium chloride was brought to shed its salt as crystals, it is seen that it differs very 



