GENESIS, HEREDITY, AND VARIATION. 339 



aggregate than units of slightly unlike forms, is tolerably 

 manifest a priori. And we have facts which prove that 

 mixing allied but somewhat different units; does lead to com- 

 parative instability. Most metallic alloys exemplify this 

 truth. Common solder, which is a mixture of lead and tin, 

 melts at a much lower temperature than either lead or tin. 

 The compound of lead, tin, and bismuth, called " fusible 

 metal/' becomes fluid at the temperature of boiling water; 

 while the temperatures at which lead, tin, and bismuth be- 

 come fluid are, respectively, 612, 44*2, and 497 F. Still 

 more remarkable is the illustration furnished by potassium 

 and sodium. These metals are very near akin in all respects 

 in their specific gravities, their atomic weights, their 

 chemical affinities, and the properties of their compounds. 

 That is to say, all the evidences unite to show that their 

 units, though not identical, have a close resemblance. What 

 now happens when they are mixed? Potassium alone melts 

 at 136, sodium alone melts at 190, but the alloy of potassium 

 and sodium is liquid at the ordinary temperature of the air. 

 Observe the meaning of these facts, expressed in general terms. 

 The maintenance of a solid form by any group of units implies 

 among them an arrangement so stable that it is not over- 

 thrown by the incident forces. Whereas the assumption of a 

 liquid form implies that the incident forces suffice to destroy 

 the arrangement of the units. In the one case the thermal 

 undulations fail to dislocate the parts; while in the other 

 case the parts are so dislocated by the thermal undulations 

 that they fall into total disorder a disorder admitting of easy 

 re-arrangement into any other order. For the liquid state is 

 a state in which the units become so far free from mutual 

 restraints, that incident forces can change their relative posi- 

 tions very readily. Thus we have reason to conclude that an 

 aggregate of units which, though in the main similar to one 

 another, have minor differences, must be more unstable than 

 an aggregate of homogeneous units. The one will yield to 

 disturbing forces which the other successfully resists. 



