396 



the eutectic mixture is allowed to consolidate the structure varies 

 somewhat according to the conditions of cooling. The substratum, so 

 to speak, is an aggregate of crystalline plates similar in optical 

 characters to those obtained by cooling fused nitre, i.e., they are 

 uniaxial and negative. This shows that nitre crystallizes as nitre in 

 the eutectic compound at a temperature 113 C. below its normal fusing 

 point. The chromate of potassium is sometimes distinctly recognizable 

 in the form of minute, yellow microlites enclosed in the nitre ; more 

 frequently, however, it cannot be recognized as occurring in distinct 

 microlites. In the latter case it is distributed through the nitre in 

 yellowish anastomosing veins or in minute strise which are often only 

 distinctly visible under a very high power. The distribution of the 

 minute stria? appears to be determined, at any rate in places, 

 by the internal structure of the nitre ; i.e., the stria? lie along 

 crystallographic planes. A study of the microscopic structure of the 

 eutectic mixture of nitre and potassium chromate leads, therefore, to 

 the conclusion that, notwithstanding the fact that it has a constant 

 composition and a constant solidifying point, it is in reality a mixture 

 and not a chemical compound nitre crystallizes as nitre and chromate 

 of potassium, at any rate under certain conditions, as chromate of 

 potassium. If, instead of taking the eutectic mixture, we take one in 

 which there is an excess of the potassium chromate, the phenomena 

 of consolidation are varied in the most interesting manner. Large 

 yellow crystals and groups of crystals of potassium chromate are first 

 formed and then the eutectic mixture consolidates. In the microscopic 

 preparations the structure is porphyritic, the eutectic mixture playing 

 the role of ground-mass. We have now considered at sufficient length 

 the general phenomena of solution and fusion, and we may summarize 

 the more important results as follows : 



1st. All homogeneous liquid masses containing two or more definite 

 compounds may be regarded as solutions. 



2nd. A solution is said to be saturated with any particular com- 

 pound when a change in temperature (generally a lowering) 

 causes the crystallization of that compound. 



3rd. A solution is said to be " super-saturated " for any particular 

 compound when the temperature is lower than that at 

 which the compound would separate if crystals of that 

 compound were present. 



4th. A liquid is said to be over-cooled when its temperature is 

 lower than that of its normal consolidating point. Over- 

 cooling is, therefore, analogous to super-saturation. 



5th. If the different compounds in a solution are present in eutectic 

 proportions the mass will consolidate at a fixed temperature. 



6th. If such a mass crystallizes, in the act of solidifying, two or 

 more compounds will be formed, and it is probable that 

 these compounds may form ultra-microscopic, microscopic or 

 macroscopic aggregates. 



