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POPULAR SCIENCE MONTHLY. 



oxygen contain, for each unit weight of nitrogen, 0.57, 1.14, 1.72, 2.29 

 or 2.86 unit weights of oxygen.* Between these combinations there 

 is no intermediate proportion. This peculiarity is characteristic of 

 chemistry in contradistinction to physics, where the more simple con- 

 tinuous and gradual transition from one state to another prevails. 

 This difference between the two sister sciences has often caused con- 

 troversies in the domain of physical chemistry. The occurrence of 

 discontinuous changes and of multiple proportions has frequently been 

 assumed, when a closer investigation has found nothing of the sort. 



The law of multiple proportions is the one fundamental conception 

 upon which modern chemistry is built up. Another is the law of 

 Avogadro, which asserts that equal volumes of different gases under 



like conditions of temperature and 

 pressure contain the same number 

 of molecules. This conception, 

 dating from the beginning of the 

 nineteenth century, was at first 

 strongly combated, and it was its 

 great value in explaining the new 

 discoveries in the rapidly growing 

 domain of organic chemistry which 

 led to its general acceptance in the 

 middle of the past century, after 

 Cannizzaro had argued strongly in 

 its favor. 



There were, however, some diffi- 

 culties to be removed before Avo- 

 gadro's law could be accepted. For 

 - instance, it was found that the 

 molecular volume of sal-ammoniac, 

 NH 4 C1, in the gaseous state was 

 greater than might be expected 

 from its chemical composition. 

 This led to the supposition that the 

 molecules of sal-ammoniac when in 

 the gaseous state are partially decomposed into ammonia, NH 3 , and hy- 

 drochloric acid, HC1. Indeed v. Pebal and v. Than succeeded in show- 

 ing that this really happens. They used an apparatus that is shown in 

 the annexed figure (Fig. 1). Two coaxial tubes are placed the one 

 inside the other by means of a cork. The outer tube was closed at its 

 upper end ; the inner one was open and contained at C a diaphragm of 

 asbestos and above that a piece of sal-ammoniac. The upper end was 



* To explain this we suppose, in accordance with Dalton, that the molecules 

 of the different combinations of nitrogen with oxygen contain two atoms of 

 nitrogen and one, two, three, four or five atoms of oxygen. 



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Fig. 1. 



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