Laboratory—No.3

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AffinityEvery science, every art, every trade, has peculiar expressions or terms to signify certain things, certain effects, and certain results; but in everyday life these things are differently expressed, or have a different meaning Thus the word pie, to you and me, dear reader, means some viand, savory or sour, as the case may be ; but the printer uses the word pi to designate a pellmell of type, all of a heap or wrongly placed, thus ri aeu m apior A miner's name for a basket is a corf or corve ; a painter calls a paintbrush "a tool" Similar technicalities will be found in every branch of industry, and learning them is one part of the duty of an apprentice In chemistry we have the word affinity, which means a liking of one substance for another, so that it will unite, forming by this union a body which is neutral between the two, whose affinity for each other causes their combination Thus, caustic potash has a strong affinity for nitric acid, which is also very caustic and corrosive, but when combined, the harmless material, saltpeter (nitrate of potash), is the result It is the affinity of iron for the oxygen in the air which causes it to rust (to produce oxyd of iron) ; and to prevent iron from rusting we cover it with oil, which, acting as a varnish, shuts out the oxygen Gold does not rust, because it has no affinity for oxygen ; so no coating of oil is necessary in this case Some substances have a greater affinity for a particular material than the body with which it may be already united; when such is the case, and the addition of the former substance be made, the two bodies having the greatest affinity for each other unite, and that which has the least affinity is displaced This is illustrated when tartaric acid is added to carbonate of soda ; the tartaric acid unites with the soda and turns out the carbonic acid, as is seen in mixing a seidlitz powder Carbonic acid being a gaft causes the effervescence If you have only affinity for philosophy, you will read this with attention; if not, there is no affinity between our ideas

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