154 SCIENCE PROGRESS 



Chemical Society of London in 1872, and entitled Considerations 

 on some Points of the Theoretic TeacJiing of Chemistry. 



A congress of chemists was called at Carlsruhe, in i860, to 

 endeavour to bring some order into the confusion which pre- 

 vailed in theoretical chemistry. Copies of Cannizzaro's y^/^5/r(7c/ 

 were distributed among the members of the Congress. Lothar 

 Meyer has told how, when he read the pamphlet, after the 

 Congress : 



" Es fiel mir wie Schuppen von den Augen, die Zweifel 

 schwanden, und das Gefiihl ruhigster Sicherheit trat an ihre 

 Stelle." 



Cannizzaro begins by setting forth the historical fact that 

 almost every chemist who came after the appearance of Dalton's 

 book had been forced to think about the actions he studied in 

 images suggested by the atomic theory. He then reasserts 

 Avogadro's hypothesis — equal volumes of gases, whether simple 

 or compound, contain equal numbers of molecules — and refuses 

 to admit any exceptions to this statement, declaring that 

 apparent exceptions will be found not to be real exceptions 

 when more searching experiments shall have been made. By 

 affirming the universal applicability of the Avogadrean sup- 

 position, Cannizzaro said that results are obtained which are 

 in keeping with all the laws that have been formulated by 

 chemistry and physics. Cannizzaro then restates Avogadro's 

 method of determining molecular weights ; it is only necessary, 

 he said, to find how man}'^ times any specified elementary or 

 compound gas is heavier than hydrogen. The values which 

 had been determined for molecular weights by this method had 

 been so out of keeping with those arrived at by chemical 

 methods, and had led chemists into so many perplexities, that 

 the Avogadrean method had been either abandoned or used 

 only in particular cases and as subsidiary to other, less direct 

 (and, as most chemists thought, less theoretical) methods. By 

 a stroke of genius, Cannizzaro changed the unit to which mole- 

 cular weights determined from measurements of vapour densities 

 are referred. He said : 



" Instead of taking for your unit the weight of an entire 

 molecule of hydrogen, take rather the half of this weight, that 

 is to say, the quantity of hydrogen contained in a molecule of 

 hydrochloric acid." 



