506 Wisconsin Academy of Sciences , Arts, and Letters. 
by experiment whether there was anything in the idea of the 
Walden inversion i. e. whether by nse of certain reagents, he 
might obtain, not the corresponding substituted product, but its 
optical antipode. This could only be determined by careful 
experiment, since the sign of actual rotation whether dextro or 
laevo, is no positive indication as to whether the body is really 
a (d) or an (1) derivative. Fisher not only confirmed the 
work of Walden by showing that an analagous inversion takes 
place with a brompropionic acid by the successive action of phos¬ 
phorus pentachloride and moist silver carbonate, but he also 
discovered another case where the change was accompanied by 
inversion, namely the conversion of optically active alanin into 
brompropionic acid, using nitrosyl bromide (H'OiBr) as the 
brominating agent. 
COOH COOH 
I I 
HC— NH 3 + NOBr- > BrC—H + N 2 + H s O 
i r 
CH 3 ch 3 
d alanine 1 brompropionic 
Fisher rigidly established that the Walden inversion was 
confined to these two phases : Phase 1—replacement of Cl by 
OH by means of silver oxide or other bases of the same type. 
Phase 2—replacement of amino (NH 2 ) group by halogen, using 
nitrosyl halide as agent. That the action of phosphorus penta¬ 
chloride was normal, while that of nitrosyl bromide was ab¬ 
normal, he established as follows: 
d iactic acid + PBr 5 -d brompropionic acid (dextrorotary) 
d lactic ethyl ester + PBr 5 - >■ d brompropionic ethyl ester (dextroro¬ 
tatory) 
d alanin + NOBr - > 1 brompropionic acid (laevorotatory) 
d alanin ethyl ester + NOBr - > d brompropionic ethyl ester (dextro¬ 
rotatory). 
Other evidence of course was furnished, but the main point 
in Fisher’s proof was the comparison of results obtained by 
various reagents on the free acids and their corresponding 
esters. If these two actions were analagous, the substituting 
agent reacted normally; if different products were obtained in 
the two actions, an inversion must have occurred in the case of 
