210 Dr. Play fair on Transformations 



molecules are peculiarly liable to change. If it once be ad- 

 mitted that an assisting affinity may exist in the sense defined 

 in the present paper, then we see the same cause operating 

 upon organic as well as inorganic molecules. When nitric 

 acid acts on oxalic acid or starch, an inorganic body (a pro- 

 tosalt of manganese) lowers the temperature necessary for 

 the oxidation, and exerts its influence until all the starch 

 is converted into carbonic acid, being equally efficacious 

 on the addition of more nitric acid and starch. Here the 

 body acting as an assistant remains unchanged, and there- 

 fore continues its action ad infinitum, rendering it impossible 

 to prepare oxalic acid from nitric acid and starch or sugar, 

 carbonic acid being the only product*. Had the assistant 

 oxidizer passed from solution during the progress of the oxida- 

 tion, it could not of course continue its favourable effect, and 

 a new portion of it must have been added. Here the inor- 

 ganic salt enables the sugar to oxidize itself from the sur- 

 rounding medium just as yeast does, the only difference being 

 that the yeast itself suffers change, and therefore can only 

 continue its action for a limited period. It is exactly in the 

 same condition as a mixture of nitric acid and binoxide of 

 nitrogen made to act on protochloride of tin. A. small portion 

 of the latter added to such a mixture is oxidized, but when 

 the solution is heated until all the NOg is expelled, oxidation 

 does not ensue on the addition of a new portion at the same 

 low temperature as before. Now Saussure and Colin have 

 shown that j^east only induces fermentation when it is in a 

 position to absorb oxygen. It acts therefore strictly as bin- 

 oxide of nitrogen, or a protosalt of manganese, in the previous 

 instances, by adding its affinity for oxygen to that of the 

 sugar, the added affinities of both completing the union. The 

 only difference between these two decompositions is, that in 

 one case the oxidizing agent is nitric acid, in the other it is 

 water. The composition of sugar shows it to contain the 

 elements of alcohol and carbonic acid minus an atom of water. 

 In such a compound we have the affinity of carbon for hy- 

 drogen and of carbon for oxygen. The yeast by its nitrogen 

 also exerts an affinity for hydrogen, and by its carbon for 

 oxygen. The united affinities of the sugar and of the yeast 

 acting upon water decompose it, its elements on their libera- 

 tion being shared by the carbon of the sugar, for which it 

 may be supposed to have the strongest affinities, Cjg IIn On 



• In this it resembles the action of oxalic acid in converting an unlimited 



quantity of oxamide into oxalate of .ammonia, with this difference, that the 

 oxalic acid, which causes the change, may not be the same, but a regene- 

 rated portion, while the salt of manganese always remains unchanged. 



