MICROCHEMIS TRY. 6 1 



Bubbles, as can be directly observed under the microscope. 

 It has been pointed out by Melnikoff (I, 30) that a concen- 

 trated acid* should be used for the recognition of small 

 quantities of carbonic acid, and that care should be taken 

 that it reaches the bodies to be tested as quickly as possible. 

 Evidently, the more slowly the evolution of carbonic acid 

 occurs, the more readily will it be absorbed by the surround- 

 ing water and carried away by diffusion without being given 

 off in bubbles. 



91. For the recognition of calcium a solution of ammo- 

 ninm oxalate acidified with acetic acid may be used. 



The manner in which this solution reacts with calcium 

 salts is largely dependent upon its strength. For example, 

 I obtained, in sections of the leaf of Ficus elastica, abundant 

 masses of crystals grown together in gland-like masses within 

 and near the cystolith cells, by placing them in a solution 

 containing .5$ of ammonium oxalate and \% of acetic acid. 

 This reaction took place at once on placing the sections in 

 the solution, which had previously been heated to boiling. 

 The crystals thus formed are strongly doubly refractive. 



But when a solution containing 10$ of ammonium oxalate 

 and \% of acetic acid was used, the oxalate was precipitated 

 directly in the cystoliths, which appeared quite unchanged 

 on microscopic examination. It was only when the sections 

 were placed in the boiling solution that the cystoliths 

 showed a more or less granular structure on their surfaces. 

 The presence of a crust of calcium oxalate on the cystoliths 

 can be easily shown by placing the sections in pure 10$ 

 acetic acid, after washing out the ammonium oxalate. The 

 still unchanged calcium carbonate incrusting the nucleus of 

 the cystolith is then dissolved with the formation of abun- 

 dant bubbles of gas, while the crust of calcium oxalate re- 

 mains undissolved. By the subsequent addition of hydro- 

 chloric acid the latter is also dissolved, so that the pure 

 cellulose skeleton of the cystolith alone remains. 



* Concentrated HC1 is best. Concentrated acetic acid cannot be used, 

 since it dissolves calcium carbonate more slowly than dilute acid. 



