276 THE INDUCTIOISS OF BIOLOGY. 



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 comparative in- 

 stability. Most metallic alloys exemplify this truth. Com- 

 mon 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 become fluid, are, respectively, 

 612^, 442^, and 497^ F. Still more remarkable is the illustra- 

 tion furnished by potassium and sodium. These metals are 

 very near akin in all respects — in their specific gravities, their 

 atomic w^eights, their chemical afiinities, 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 resem- 

 blance. 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 stablo, that 

 it cannot be overthrown 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 positions very readily. Thus we have reason to 

 conclude, that an aggregate of units which, though in the 

 main similar to each other, have minor difierences, must be 

 more unstable than an aggregate of homogeneous units : the 

 one will yield to disturbing forces which the other successfully 

 resists. 



