Polytechnic Association Proceedings. 767 



This oxide is a feathery mass, and is deposited in the form of a coat- 

 ing on the foil, so rapidly that its increase is visible to the naked 

 eye. Aluminum has so strong an affinity for oxygen that it has 

 been found extremely difficult to separate the metal from common 

 clay, which is nearly pure silicate of alumina; but when once sepa- 

 rated aluminum does not readily unite with oxygen. It does not 

 easily tarnish in air, and will not decompose water. Being a good 

 conductor of heat and electricity the essential conditions for rapid 

 combustion do not occur, but when the pores of the metal have 

 been filled with mercury, the particles of aluminum in a separate 

 state cannot resist the attack of the oxygen in common air, and the 

 accumulated heat generated by the rapid oxidation of the surface 

 is soon high enough to insure the oxidation of the inner portion of 

 the metal, which it is supposed, the mercury has not reached. Dr. 

 Wurtz does not thus account for the phenomenon ; he believes 

 quicksilver produces an allotropic modification of the metal by 

 which it is brought from a passive into an active state; and sug- 

 gests that it is possible to reverse this action in the case of ele- 

 ments which are easily oxidized, so that strong electro-positive 

 metals like sodium and potassium, by being brought into a passive 

 condition, could be handled and exposed to common air without 

 danger. He concludes that ordinary aluminum is in a passive state 

 resembling that of iron, which, when in contact with platinum, 

 resists the action of nitric acid; and he maintains that iron may be 

 permanently held in that condition. When he succeeds in making 

 one powerful electro-positive metal inert in the presence of oxygen 

 we shall gladly accept his theory. 



HUMAN DECADENCE. 



In an able thesis on death, Dr. Acosta, of Paris, discusses the 

 difficulty of determining the commencement of old age, and says 

 the Greeks regarded the age of forty-nine (seven times seven, their 

 climacteric number) as the culminating point of human strength. 

 Another French writer, M. Flourens, however, holds that deca- 

 dence does not commence until the seventieth year. The Chinese 

 call men who have attained that age "rare birds," and those 

 who reach the ninetieth year " old loiterers." The two climac- 

 teric ages of the Arabs w^ere sixty-three (seven times nine) and 

 eighty-one (nine times nine). The first was considered the grand 

 climacteric among the ancients, and those who passed it were accus- 

 tomed to congratulate each other. Physiologists recognize the 



