PHYSICAL CHEMISTRY 543 



movement of the surface of a water manometer in contact 

 with the gas. By photographing the region near the point of 

 entrance of the hght, with the aid of a second spark, he showed 

 that no appreciable expansion occurred until yvij second after 

 illumination. The disturbance was at a maximum in ^V second, 

 and had entirely disappeared in tu second. The conclusion is 

 drawn that the reaction between hydrogen and chlorine reaches 

 a maximum ^V second after illumination by a chain-like 

 mechanism similar to that postulated by Bodenstein and 

 Nernst. Since one quantum of energy converts 10* molecules 

 of the mixture into hydrochloric acid, it can be shown that 

 each element in the chain takes io~* seconds to be completed. 

 This interval of time occurs frequently in modern theory of 

 chemical change. An extension of this investigation should 

 prove of considerable value in the elucidation of the processes 

 occurring during the induction period. 



Physical Chemistry of Foodstuffs. — An important advance 

 has been made by Paul, Dietzel, and Taufel (Z. Elektrochem., 

 1922, 28, 435-6) in the application of the methods of psycho- 

 physics to determine the relationship between acid taste and 

 hydrogen-ion concentration. By means of these methods, 

 and the introduction of new terminology, the measurement of 

 acid taste has been placed on a quantitative basis. Molecular 

 acidity, for example, is defined as the number of mols of 

 hydrochloric acid, dissolved in a fixed volume of water, which 

 tastes as acid as one mol of the acid substance dissolved in 

 the same volume of water. The molecular acidity is deter- 

 mined not entirely by the hydrogen-ion concentration, but 

 mainly by the capacity of the acid substance to yield its reserve 

 of hydrogen ions to the tongue. The nature of the anion 

 influences to some extent the acid taste of a substance. It 

 is possible to arrange acid substances in a series of increasing 

 molecular acidities, thus carbonic acid, potassium hydrogen 

 tartrate, acetic acid, lactic acid, acetyl lactic acid, hydro- 

 chloric acid, and tartaric acid form such a series. Carbonic 

 acid possesses the weakest, and tartaric acid the strongest, 

 acid taste in this series. This arrangement is not in agreement 

 with the dissociation constants of the acids. Thus the two 

 acids, acetyl lactic and tartaric acids, although possessing 

 very similar dissociation constants, possess very different 

 acidities. Many analogies appear to exist between the two 

 properties, specific acidity and specific sweetness. Both in- 

 crease, for example, with concentration in a similar manner. 

 This relationship between two of the taste qualities suggests 

 that similar regularities may be found between these and the 

 salt, and bitter qualities of substances. 



The Adsorption and Fluorescent Spectra of Aromatic Hydro- 



