244 PHYSIOLOGY OF NUTRITION. 



vessel a slab of marble in the manner described in 25. The 

 fluid in a remains clear, owing to the fact that the marble 

 becomes quickly coated with a crust of Calcium oxalate, which 

 retards the action of the Oxalic acid. The fluid in 6, on the other 

 hand, rapidly becomes very turbid, which can only be explained as 

 due to the following" reaction. The Oxalic acid decomposes the 

 Sodium chloride. The marble is acted on by the liberated Hydro- 

 chloric acid with formation of Calcium chloride. On this the 

 Oxalic acid now acts, and the Calcium oxalate so formed, which 

 rapidly collects in large quantities at the bottom of the vessel, 

 causes the turbidity of the fluid. 



We can satisfy ourselves in still another way that organic acids 

 are able to decompose chlorides. 1 Six culture fluids are prepared : 

 a, 15 c.c. of distilled water ; 5, 15 c.c. of water containing O020 gr. 

 of Citric acid; c, 15 c.c. of water containing O7 gr. of Potassium 

 chloride ; d, 15 c.c. of water containing O7 gr. of Sodium chloride ; 

 e, 15 c.c. of water containing O020 gr. of Citric acid, and 0'7 gr. 

 of Potassium chloride ; /, 15 c.c. of water containing O020 gr. 

 of Citric acid, and O7 gr. of Sodium chloride. The fluids are 

 now left standing for about twenty-four hours, and then to each 

 we add a few drops of a very dilute aqueous solution of methyl 

 violet. The fluids a, 6, c, and d, exhibit almost the same violet 

 tint ; e and /, on the other hand, are distinctly blue in colour. This 

 indicates the presence of free Hydrochloric acid, since, while very 

 dilute solutions of Citric acid scarcely affect the colour of methyl 

 violet, it becomes blue in presence of very dilute Hydrochloric 

 acid.* 



1 See Detmer, Botan. Zeitung, 1884, No. 50. 



92. Absorptive Capacity of the Soil. 



It is a highly important fact that the soil is able to retain very 

 energetically (to absorb) a number of substances with which it 

 comes into contact. Potash, ammonia, and Phosphoric acid are 

 most actively absorbed by the soil, and are therefore prevented 

 from sinking deeply into the ground, a fact which is obviously of 



* Giinsberg's test for Hydrochloric acid in presence of chlorides is also very 

 serviceable here. We mix 1 gr. of vanillin and 2 gr. of phloroglucin with 30 gr. 

 of alcohol. A few drops of this mixture, and a few drops of the fluid under 

 investigation, are placed in a white porcelain dish. If a red colour appears on 

 heating, it indicates the presence of free Hydrochloric acid. . 



