PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 443 



we were to make' an aquarium, we should first put into it a fish. 

 It would soon require fresh air. Then put in a water plant, and 

 it will absorb from the water the carbonic acid gas thrown oiF by 

 the lungs of the fish, and in return would give off oxygen. But 

 soon the water would become turbid from the presence of micros- 

 copic infusoria and plants, and we must introduce some animal to 

 eat them up. Soon we should find our aquarium overstocked ; 

 and we must introduce some animal to eat up the surplus popula- 

 tion. And finally we should have to introduce some animal to 

 perform the part of public scavenger, and eat up the refuse. An 

 aquarium thus peopled has lasted eight or ten years, and may 

 perhaps last forever. It is this balance of life, that keeps the 

 waters of the ocean pure, and not the agitation of the waves. In 

 the very first record of life, in these ancient rocks, reposing 

 immediately upon primitive rocks, we find animals forming this 

 complete circle of life. The trilobites were the scavengers of 

 those early seas, as the lobster is the scavenger of the present 

 seas. So abundant were the animals of that early age that there 

 are rocks twenty or thirty feet in thickness composed of their 

 shells cemented together. There is a mass of rock twelve feet 

 thick, extending across Western New York into Canada ; and it 

 is impossible to break off a piece of it as large us a butternut 

 without breaking a well developed shell. 



PLATING OF METALS. 



Mr. Stetson. — The placing of one metal upon another so that 

 there shall be established a connection between them may be 

 accomplished by very obvious means, such as cementing or glue- 

 ing. Common gilding is often done in that way, by putting the 

 gold leaf upon a varnish ; and that accomplishes that problem. 

 The same thing may be done, if desired, upon metals. But it is 

 very obvious that that is a very imperfect means of joining them 

 for practical purposes, because any agency that wilT destroy or 

 dissolve the cement will destroy the union between the two 

 naefrals. A similar process is that of soldering. We use a metal 

 fusible at a low temperature in the place of cement ; and here too 

 any agency, such as heat, which will destroy or soften the solder 

 will cause it to lose its tenacity, and its value ceases. Another 

 method of plating, is by covering the surface of the article to be 

 plated with an amalgum of a metal. Gold or silver amalgum, for 

 instance may be applied to brass or copper, and upon exposing 



