crystals of tourmaline, topaz, and 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 definite relation existing between the intensity of this agent and the crystal 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 Russian 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 FACTORS OF ORGANIC EVOLUTION. |
By HERBERT SPENCER.
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 implication is that the processes which 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 unusual 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,
