10 THE ATMOSPHERE. 



formed by this agent, and hence its name, which signifies 

 the former of acids. 



The fourth and last change, frequently witnessed in 

 the substance of the apple, is putrefaction. If vinegar 

 be freely exposed for a long time to a warm atmosphere, 

 it becomes putrid. The ultimate or final effect of fer- 

 mentation, upon all animal and vegetable matter, is, decay, 

 or an entire decomposition. 



Notwithstanding every species of matter in the whole 

 animal and vegetable kingdoms, is liable to undergo the 

 four changes above described, certain circumstances will 

 entirely prevent either. These circumstances are, seclu- 

 sion from the air or moisture, and more than a moderate 

 degree of heat or cold. 



By confining the flesh of quadrupeds, of fowls, or of 

 fishes, in tin canisters, perfectly air tight, it is kept for 

 years in a fresh state, without injury. 



Fresh animal food has been frequently preserved in this 

 way during a three years' voyage at sea. Any animal or 

 vegetable substance, if made perfectly dry, will undergo 

 no fermentation and no decay. The same is true if kept 

 in a frozen state, or frequently raised to a temperature 

 near boiling heat. 



IV. Oxigen acts upon all the metals. By this action 

 the rust of iron, the dross of lead, the corrosion of cop- 

 per, brass, and silver, and the most beautiful paints, such 

 as red and white lead, chrome, yellow and blue vitriol, is 

 produced. 



Some of the metals combine with oxigen rapidly, others 

 gradually, or even with difficulty. Iron, for example, is 

 very liable to rust, or oxidate, as chemists say ; while 

 gold utterly refuses to combine with oxigen, as it does 

 with most other substances, except by an indirect pro- 

 cess. If lead be kept in a melted state for a short time, 

 it absorbs so much oxigen as to be changed wholly into 

 dross, which is an oxide lead. But gold may be kept in 

 a state of fusion and motion for months, or years, and 

 never admit a particle of oxigen into its connexion ; so 

 proud a substance as gold, may, however, by an indirect 

 process, be led to combine with oxigen, and after that 

 with many other substances, When oxigen once gets 



