54 THE POPULAR SCIENCE MONTHLY. 



crystals of tourmaline, topaz, arid calamine. The ends which show 

 this peculiarity alternately exhibit positive and negative electricity 

 the one kind when the mineral is heating, and the other while it is 

 cooling. The experiments of Faraday and Tyndall also indicate this 

 causal connection. Thus the problem of crystallization may be said 

 to have arrived at the stage of a partial solution, and the" manner in 

 which the result has been obtained clearly shows why an agent like 

 electricity is the cause of crystallization ; it also shows a perfect defi- 

 nite relation existing between the intensity of this agent and the crys- 

 tal form. When it is considered that difference in crystal form is, as 

 a rule, associated with difference in chemical composition, it is easy to 

 conceive how profoundly important this relation is in the chemism of 

 substances. The intimate causal connection between electricity and 

 chemical affinity is well accepted. 



The law of the periodicity of the elements, discovered by the Rus- 

 sian chemist, Mendeljief ; the investigations of Kekule on the aromatic 

 compounds, which throw a strong light upon their structure ; the law 

 of Dulong and Petit, as to the constancy of the relation between the 

 heat and atomic weight of the elements all these give just grounds 

 for the remark that, when brought into proper connection with the 

 stated law of crystallization, an epoch may result in our knowledge 

 of atoms. 







THE FACTOES OF OEGANIC EVOLUTION. 



By HEEBEET SPENCEK. 

 II. 



THE growth of a thing is effected by the joint operation of certain 

 forces on certain materials ; and when it dwindles, there is either 

 a lack of some materials, or the forces co-operate in a way different 

 from that which produces growth. If a structure has varied, the im- 

 plication is that the processes w^hich built it up were made unlike the 

 parallel processes in other cases, by the greater or less amount of some 

 one or more of the matters or actions concerned. Where there is un- 

 usual fertility, the play of vital activities is thereby shown to have 

 deviated from the ordinary play of vital activities ; and conversely, if 

 there is infertility. If the germs, or ova, or seed, or offspring partially 

 developed, survive more or survive less, it is either because their molar 

 or molecular structures are unlike the average ones, or because they 

 are affected in unlike ways by surrounding agencies. When life is 

 prolonged, the fact implies that the combination of actions, visible and 

 invisible, constituting life, retains its equilibrium longer than usual in 

 presence of environing forces which tend to destroy its equilibrium. 

 That is to say, growth, variation, survival, death, if they are to be 

 reduced to the forms in which physical science can recognize them, 



