DECOMPOSITIONS EFFECTED BY LIGHT. 25 



in an atmosphere of carbon dioxide, consume a portion of their sugar 

 content, gaseous carbon dioxide being evolved in the process and 

 alcohol formed. The presence of this latter substance in the cells 

 of the fruit substance can be proved by distillation, as much as 

 one per cent, by weight having been detected in this way. 



PASTEUR (IV. and Y.) also studied this phenomenon, which he 

 employed as one of the main buttresses of his previously formed 

 theory asserting fermentation to be a universal phenomenon, not 

 dependent on certain organisms, but occurring in every living vege- 

 table cell debarred from a supply of oxygen. This spontaneous 

 fermentation appeared to form a striking proof of the correctness 

 of this theory, but the hopes thereby raised proved vain, since 

 Pasteur was himself the one to discover that alcohol (though in 

 merely minute quantities) is also formed in fruits when they are 

 exposed to the air. 



21. Decompositions Effected by Light and Air. 



To the chemist who is obliged to store in the dark and in 

 properly stoppered receptacles various inorganic reagents and 

 normal solutions which he desires to maintain in an unaltered 

 condition it will not be surprising to learn that sterile solutions 

 of organic substances such, for example, as are stored for use as 

 nutrient media for bacteria also gradually undergo slight modifi- 

 cations when air and light find admittance thereto. The proof 

 that these changes are of a purely chemical nature lies in the fact 

 that they do not occur when the causes indicated are absent. 



In many instances the oxidation of the medium effected in this 

 manner exerts a favourable influence on the development of the 

 organism subsequently inoculated therein. Such, for example, is 

 the case with beer-yeast. In wort through which air has been 

 blown (" roused ") for some time, the yeast sown therein develops 

 more rapidly, and deposits more quickly and satisfactorily ("breaks" 

 better) at the conclusion of fermentation, a circumstance highly 

 desired by the brewer. Sunlight possesses an even greater decom- 

 posing power than that of atmospheric oxygen. On this point we 

 are indebted to E. DUCLAUX (I.) and W. SBEKAMP (I.) for ex- 

 haustive researches, the former of whom found that, in presence 

 of air, a sterile solution of tartaric acid is split up by sunlight into 

 formic acid, carbon dioxide, and water, according to the equation 



C 4 H 6 6 + 30 = 2CH 2 2 + 2CO 2 + H 2 0. 



In a second communication (II.) on this subject the same 

 worker showed that glucose and lactose in a sterile alkaline solu- 

 tion gradually decompose into alcohol and carbon dioxide on 

 exposure to sunlight, even when oxygen is excluded. The same 

 products are yielded by them when fermented with yeast. When 

 baryta (BaH 2 2 ) or lime (CaH 2 2 ) was substituted for the alkali, 



