DISCOVERY BY ASKING QUESTIONS AND TESTING THEM. 513 



of it had been converted into a black powder, weighing in 

 some cases above two ounces. Now it was found in all 

 cases that the weight of the tin had increased, and the 

 increase of weight was always exactly equal to the dimi- 

 nution of weight which the air in the retort had under- 

 gone, measured by the quantity of new air which rushed 

 in when the beak of the retort was broken, minus the air 

 that had been driven out when the tin was originally 

 melted before the retort was hermetically sealed. Thus 

 Lavoisier proved by the first experiments that, when tin 

 is calcined in close vessels, a portion of the air of the 

 vessel disappears, and that the tin increases in weight just 

 as much as is equivalent to the loss of weight which the 

 air has sustained. He therefore inferred, that this portion 

 of the air had united with the tin, and that calx of tin is 

 a compound of tin and air.' 1 



Dr. Black asked himself, why is it that limestone 

 becomes so changed in property by being heated to red- 

 ness ? and then proceeded to test that question. He 

 weighed the limestone both before and after heating it, 

 and also the amount of moisture expelled ; but the loss of 

 water would not wholly account for the loss of weight. 

 He then thought of Dr. Hales' experiments of driving air 

 out of substances by means of heat ; he put the two ideas 

 together, and knowing that acids expelled gas from un- 

 burned limestone, he added some acid and water to the 

 unburned limestone in a bottle, and collected the evolved 

 gas in an inverted vessel filled with water, and thus 

 obtained ' fixed air,' now termed c carbonic anhydride.' 

 He weighed this gas, and found that it exactly agreed with 

 the missing weight which had occurred on heating the stone. 

 It is a very probable hypothesis that a connection 

 exists between magnetism and chemical power ; and it is 



1 Thomson, History of Chemistry, vol. ii. p. 102. 

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