434 ^^^ POPULAR SCIENCE MONTHLY. 



biology. The psychologist and the theologian are apt to ignore too 

 much the material basis of sociolog}', the organic laws which through 

 our material nature impress themselves upon the structure and develop- 

 ment of society ; while the materialist and the political economist are 

 apt to overlook or belittle the importance of the other essential cor- 

 porations, especially the church, and make sociology nothing more than 

 the highest of the material sciences. But no steady, safe progress in 

 social organization can be made unless we fully recognize the coordi- 

 nate value of these three, nor in the science of sociology unless we ap- 

 proach the subject from these three sides. 



-♦•♦- 



THE CKYSTALLIZATION OF GOLD, SILVER, AND 



OTHEE METALS. 



By THOMAS J. GEEGAN. 



THERE are tew chemical experiments so well known as the growth 

 of the " lead-tree " and " silver-tree." These carrj' our minds 

 back to the times of the alchemists, who called the first " Arbor Sa- 

 turni," and the second "Arbor DianEe," and they may be looked upon 

 as the type of a large number of phenomena in which the salt of one 

 metal in solution is decomposed by some other metal. 



My assistant, Dr. Hand, and myself have lately been experimenting 

 on these replacements, the metallic crystals which are thus produced, 

 and the forces that act through the liquid. Our more special attention 

 has been given to the mutual action of copper and nitrate of silver. 

 If these two substances be brought into contact by the intervention of 

 water, there grows on the red metal what may be called " trees," and, 

 though the analogy between the crystals and the plants is a very 

 superficial one, still the resemblances of external form are sufficiently 

 striking, and a nomenclature drawn from the garden seems the most 

 expressive. 



A microscopic view of the growth of these silver crystals round a 

 piece of copper is a truly beautiful sight ; a blue glass underneath 

 increases the efiect, but they are best seen when they reflect a strong 

 light thrown upon them. They may also be thrown upon a screen as 

 opaque objects, but the beauty and luster of their surfaces are in this way 

 lost. The crystals of silver thus produced differ both in color and form 

 according to the strength of the solution. If it be very weak — say one 

 per cent. — the copper is fringed with black bushes of the metal, which 

 in growing change color to white without any alteration of crystalline 

 form that can be detected by a powerful microscope. A stronger solu- 

 tion gives white crystals from the commencement, which frequently 



