395 



heat developed by consolidation be allowed to escape. Call this the 

 first experiment. If the proportion of nitrate of lead be greater than 

 that given above then, before the temperature of 207 C. is reached, 

 sufficient nitrate of lead will separate out to produce a solution in 

 which the two constituents are present in the same proportion as in 

 the first experiment. As more heat is withdrawn the mother liquor 

 will consolidate at the fixed temperature of 207 C. If nitrate of lead 

 be present in the original solution in smaller quantity than in the first 

 experiment, then, on cooling down to 207, nitre will separate in such 

 quantity as to produce a mother liquor containing 46'86 p.c. of nitrate 

 of lead. It follows, therefore, that if we take any liquid mixture of 

 nitre and nitrate of lead and cool it to 207, the portion remaining 

 liquid at that temperature will have a constant composition and a 

 constant solidifying point ; moreover, the solidifying point of the 

 mixture will be lower than that of either of the constituents taken 

 separately. Dr. GUTHRIE'S researches prove, therefore, that fused 

 mixtures are strictly analogous to aqueous solutions. Seeing that any 

 two substances which are capable of forming together a homogeneous 

 liquid will, when mixed in a certain proportion, consolidate simultane- 

 ously at a fixed temperature which is lower than that at which 

 either of the two substances can be melted when taken separately, 

 Dr. GUTHRIE proposed that a mixture (or compound) of the two 

 substances in the required proportion should be termed an eutectic 

 mixture (or compound) from ev rrjKeiv, to melt easily. Now, if a drop 

 of the solution of nitrate of lead in nitre, in eutectic proportions, be 

 placed on a hot glass slide under the microscope then, as the solution 

 cools, strongly double-refracting crystalloids (VOGELSANG) are first seen 

 to form in great numbers, and immediately afterwards the portion 

 remaining liquid consolidates as a confusedly crystalline mass or as a 

 fibrous aggregate often showing the most beautiful radial structures. 

 It is impossible to be certain as to the precise nature of the 

 crystalline bodies/ 1 ) but the phenomena observed do not bear out the 

 view that the mass which consolidates at 207 C. is one definite 

 chemical compound. Somewhat more positive conclusions can be drawn 

 from a study of a solution of potassium chromate in nitre. The 

 eutectic mixture of these substances contains 376 p.c. of the former 

 salt and consolidates at 295 C. If fused nitre be allowed to 

 crystallize on a microscopic slide under a cover-glass it forms an 

 aggregate of crystalline grains or rather plates. The plates mutually 

 interfere with each other, but each possesses definite optic characters- 

 Those which lie nearly at right angles to an optic axis are seen, in 

 convergent polarized light, to be uniaxial and negative. < 2) Now when 



(1) The writer of this work is responsible for the descriptions of the microscopic 

 characters of the eutectic compounds. 



(2) Nitre is dimorphous. When formed at low temperatures from aqueous solutions it 

 is orthorhombic. When formed at high temperatures from fusion or anhydrous solutions it is 

 rhoinbuhedral. 



