EIPEKIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 521 



Ordinary brass was got with difficulty by repeatedly submitting a 

 mixture of zinc and copper filings to great pressure. 



Spring lays down the rule that ' matter assumes that condition which 

 corresponds to the volume it is made to occupy.' Thus when two bodies 

 can by their union or interaction produce one of less volume or greater 

 density, this will be produced by great pressure ; and he explains by this 

 the readiness with which Woods' and Rose's alloys are formed as contrasted 

 with the formation of brass by great pressure. At 6,600 atmos. arsenides 

 of lead, copper, and silver are formed from the elements in the same way ; 

 and the following sulphides, viz., of Mg, Zn, Fe", Cd, Bi, Pb, Ag, Cu, 

 Sn, and Sb, by repeated compressions. Spring found that the extent to 

 which the formation of sulphide is carried in this process depends on 

 time as one condition ; by the lapse of time alone more of the sulphur and 

 metal combining to form more of the sulphide. 



Chemical Actions hy intense Pressure. — Solids. 



A dry intimate mixture of sodium carbonate and barium sulphate 

 being submitted to intense pressure, double decomposition occurs to a 

 considerable extent. A double decomposition also occurs under similar 

 circumstances with a mixture of dry powdered sodium sulphate and 

 barium carbonate ; and when the resulting compressed cylinder was 

 again powdered and submitted to intense pressure, and the process 

 repeated several times, the substance being maintained for some time 

 under pressure, the formation of barium suiphate increased to about 80"6 

 per cent., which seemed to be the limit. 



The Transition-point. 



J. H. Van 't Hoff calls by the name ' transition-point ' that constant 

 temperature at which a chemical change can begin and continue, and 

 compares it to the melting-point of a solid in its theoretical relations. 

 Thus, if there is a transition-point in a chemical action, this point must 

 be dependent on the pressure in such a manner that if the change is 

 accompanied by decrease of volume, as in the melting of ice, the transition- 

 temperature should be lowered by increased pressure. As an example, 

 Van 't Hofi" and Spring take the case of a double acetate of calcium and 

 copper.^ At ordinary pressures the double-salt, which is blue, is 

 decomposed into its component salts at 75°, and this change is accom- 

 panied by diminution of volume, as in the melting of ice. Therefore 

 increase of pressure should lower the transition-point, and therefore 

 promote the formation at a lower temperature than 75° of the separate 

 salts from the double salt. Now the cupric acetate by itself is green, and 

 the transition can therefore be observed by the change of colour. At 

 6,000 atmos., and temp. 16°, the finely powdered double acetate became a 

 marble-like crystalline mass, but showed no sign of decompositioxi ; at 

 40° a quantity of the mass liquefied ; on relieving the pressure it became 

 all solid, the dark blue had changed for the most part to green, and there 

 was found a coating of copper round the sides of the containing vessel. 

 At 50° the action was more rapid. It was found also that the decompo- 



> Bull. Ace. Belg. 49, 3i4 ; Zcitschr.f. j/Jnju. Chem. 1, 227; C.S.J. Abs. 1888, p. 341; 

 Rec. trav. cMm. 6, 91 ; Bcr. 1887, II. 276, 311. 



