104 PRINCIPLES OF CIIK.MJSTRY 



its homogeneity, as in solutions, whilst it is impossible to add any one 

 of the component parts to a definite chemical compound without de- 

 stroying the homogeneity of the entire mass. Definite chemical com- 

 pounds only decompose at a certain rise in temperature ; on a lowering 

 in temperature they do not, at least with very few exceptions, yield 

 their components like solutions which form ice or compounds with water 

 of crystallisation. This obliges one to consider that solutions contain 

 water as water, Mj although it may sometimes be in a very small quan- 

 tity. Therefore solutions which are capable of entirely solidifying (for 

 instance, cryohydrates 'and crystallo-hydrates i.e., compounds with 

 water of crystallisation which are capable of melting or the compound 

 of 84^ parts of sulphuric acid, H 2 SO 4 , with hH parts of water, H 2 0, 

 or H 2 SO 4 ,H 2 O, or H 4 SO^) appear as true definite chemical compounds, 

 If, then, we imagine such a definite compound in a liquid state, and 

 admit that it partially decomposes in this state, separating water 

 not as ice or vapour (for then the system would be heterogeneous. 

 including substances in different physical states), but in a liquid form, 

 when the system will be homogeneous -then we shall form an idea of 

 a solution as an unstable, decomposing fluid equilibrium between water 

 and the substance dissolved. Just as the component elements may be 

 added to a gaseous mixture without destroying its homogeneity, so both 

 the solvent may be added to a solution (the solution will then be 

 obtained diluted, and no longer presenting a definite composition), arid 

 also the substance dissolved may be added (with a solid and a saturated 

 solution a supersaturated solution will be obtained), which may, how- 

 ever, owing to the force of the cohesion of its parts, separate out from 

 the solution in a crystallised form. In adding the solvent, or the 

 substance dissolved, without destruction of the homogeneity of the 

 whole, we altered their relative quantity (the proportion of the acting 

 masses), by which there will be an alteration, both in the quantity of the 

 water, forming one of the products of dissociation, and also of the relative 

 quantity of one or many of the definite compounds between the water 

 and the substance dissolved. Owing to this change, there occurs an 

 alteration in the properties of a solution (contraction, change of vapour 

 tension, &c.) ; not in the sense of a purely mechanical change in the 

 proportion of the components (as in the intermixture of non reacting 



66 Such a phenomenon frequently presents itself in purely chemical action. Km- 

 instance, let a liquid substance A give, with another liquid substance J3, under the condi- 

 tions of an experiment, a mere minute quantity of a solid or gaseous substance C. This 

 small quantity will separate out (pass away from the sphere of action, as Berthollet 

 expressed it), and the remaining masses of A and I? will again give C ; consequently, 

 under these conditions, action will go on to the end. Such, it seems to me, is the action 

 in solutions when they yield ice or vapour indicating the presence of water. 



