OX WATKK AND ITS COMPOUNDS 105 



gases), 1'iit in the sense of an alteration in the quantity of those definite 

 liquid chemical compounds which are determined by the chemical attrac- 

 tion between water and the substance dissolved in it, and by their 

 capacity for forming with it 'liri-rxe compounds,*'" which is seen in the 

 capacity of one substance to form with water many various crystal I - 

 },,/,} rofrx, or compounds with water of crystallisation, showing diverse 

 and independent properties. From these considerations, solution 

 ma ;/ l> regarded as fluid, unstable, definite chemical compounds in a 

 state of dissociation. 



67 Certain substances are capable of forming only one compound, others several, and 

 these of the most varied degrees of stability. The compounds of water are instances of 

 tins kind. In solutions of sulphuric- ;ic-icls (nee note 19), for example, the existence must 

 l)f acknowledged of several different definite compounds. Many of these have not yet 

 In 'en obtained in a free state, and it may be that they cannot be obtained in any other 

 but a liquid form that is, dissolved; just as there are many undoubted definite com- 

 pounds which only exist in one physical state. Among the hydrates such instances 

 occur. The compound CO 2 + 8HUO (see note 31), according to Wroblewski, only occurs in 

 a solid form. Hydrates like H.,S + 1'2H 2 O (De Forcrand and Villard), HBr + H 2 O (Rooze- 

 boom), can only be accepted on the basis of a decrease of tension, but present themselves 

 as very transient substances, incapable of existing in a stable free state. Even sulphuric 

 acid, H.iSO 4 , itself, which undoubtedly is a definite compound, fumes in a liquid form, 

 evolving the anhydride, SO 5 that is, exhibits a very unstable equilibrium. The crystallo- 

 hydrates of chlorine, C1 3 + 8H 2 O, of hydrogen sulphide, H 2 S + 12H<>O (it is formed at 0, 

 and is completely decomposed at +1, as then 1 vol. of water only dissolves 4 vols. of 

 hydrogen sulphide, while at 0'1 it dissolves about 100 vols.), and of many other gases, 

 are instances of hydrates which are very unstable. 



68 Of such a kind are also other indefinite chemical compounds; for example, 

 metallic alloys. These are solid substances or solidified solutions of metals. They also 

 contain definite compounds, and may contain an excess of one of the metals. According 

 to the experiments of Laurie (1888), the alloys of zinc with copper in respect to the electro- 

 motive force in galvanic batteries behave just like zinc if the proportion of copper in the 

 alloy does not exceed a certain percentage that is, until a definite compound is attained 

 for then there are yet particles of free zinc ; but if a copper surface be taken, and it be 

 covered by only one-thousandth part of its area of zinc, then only the zinc will act in a 

 galvanic battery. 



69 According to the above supposition, the condition of solutions in the sense of the 

 kinetic hypothesis of matter (that is, on the supposition of an internal movement of 

 molecules and atoms) may be represented in the following form: In a homogeneous 

 liquid for instance, water the molecules occur in a certain state of, although mobile, 

 >till stable, equilibrium. When a substance A dissolves in water, its molecules form with 

 -cveral molecules of water, systems AnHoO, which are so unstable that when surrounded 

 by molecules of water they decompose and re-form, so that A passes from one mass of 

 molecules of water to another, and the molecules of water which were at this moment in 

 harmonious movement with A in the form of the system AnH.^O, in the next instant 

 may have already succeeded in getting free. The addition of water or of molecules of A 

 may either only alter the number of free molecules, which in their turn enter into systems 

 A n\ LO, or they may introduce conditions for the possibility of building up new systems 

 . 1 ,, H..O, where m is either greater or less than n. If in the solution the relation of the 

 molecules be the same as in the system AmH<>O, then the addition of fresh molecules of 

 w;iter or of A would be followed by the formation of new molecules ^4H 2 O. The relative 

 quantity, stability, and composition of these systems or definite compounds will vary in 

 one or another solution. Such a view of solutions came to me from a most intimate 

 study of the variation of their specific gravities, to which my book, cited in note 19, is 



