In applying the theorem of Le Chatelier to the course of the solubility curve, it should be noted that by heat of solution there is meant, not the heat effect produced on dissolving the salt in a large amount of solvent (which is the usual signification of the expression), but the heat which is absorbed or evolved when the salt is dissolved in the almost saturated solution (the so-called last heat of solution). Not only does the heat effect in the two cases have a different value, but it may even have a different sign. A striking example of this is afforded by cupric chloride, as the following figures show:[185]-- ----------------------------------------------------------- Number of gram-molecules of | CuCl_{2}, 2H_{2}O dissolved in 198 | Heat effect. gram-molecules of water. | -----------------------------------+----------------------- 1 | +37 K 2.02 | +66 " 4.15 | +105 " 7.07 | +117 " 9.95 | +117 " 11 | +91 " 18.8 | -10 " 19.6 | -31 " 24.75 | -198 " ------------------------------------------------------------ In the above table the positive sign indicates evolution of heat, the negative sign, absorption of heat; and the values of the heat effect are expressed in centuple calories. Judging from the heat effect produced on dissolving cupric chloride in a large bulk of water, we should predict that the solubility of that salt would diminish with rise of temperature; as a matter of fact, it increases. This is in accordance with the fact that {111} the last heat of solution is _negative_ (as expressed above), _i.e._ solution of the salt in the almost saturated solution is accompanied by absorption of heat. We are led to expect this from the fact that the heat of solution changes sign from positive to negative as the concentration increases; experiment also showed it to be the case. Despite its many forms, it should be particularly noted that the solubility curve of any substance is _continuous_, so long as the solid phase, or solid substance in contact with the solution, remains unchanged. If any "break" or discontinuous change in the direction of the curve occurs, it is a sign that the _solid phase has undergone alteration_. Conversely, if it is known that a change takes place in the solid phase, a break in the solubility curve can be predicted. We shall presently meet with examples of this.[186] A.--ANHYDROUS SALT AND WATER.