294 SCIENCE PROGRESS 



An analytical reaction of some importance has been dis- 

 covered by Guareschi (Atti. R. Accad. Sci. Torino, 50, 354, 

 191 5) in the action of ammonium bromide on metallic iodides. 

 All such compounds, even the very stable Agl, are decomposed 

 when heated in a long narrow hard-glass tube with ammonium 

 bromide, a reaction taking place according to the following 

 equation : 



2MI + 2NH 4 Br -> 2 MBr + 2NH3 + H 2 + I 2 . 



The method has proved capable of detecting as little as 

 0.000 1 grm. potassium iodide in presence of a large amount of 

 alkali or alkaline earth chlorides or bromides. As it is possible 

 to obtain insoluble iodides of most metals, the method is preg- 

 nant with capabilities. 



A couple of interesting papers have recently appeared on 

 the relative merits of the commoner desiccating agents, and, as 

 the results are of general utility to chemists, we may con- 

 veniently include them here. 



Berkeley and Hartley {Phil. Mag. 29, 609, 1915) claim 

 great efficiency for a surface type of sulphuric acid drier. Sul- 

 phuric acid would seem to be as capable of drying air as phos- 

 phorus pentoxide for amounts of moisture of the order of the 

 ordinary atmospheric vapour pressure. Pure anhydrous cop- 

 per sulphate, strangely enough, is very effective if the air only 

 contains traces of moisture — amounts less than the vapour- 

 pressure of the hydrate CuS0 4 . H 2 0. 



In the ordinary desiccating jar under reduced pressure, 

 Marsden and Elliot (/. Ind. Eng. Chem. 7, 320, 191 5) find 

 aluminium oxide more efficient than either sulphuric acid or 

 calcium chloride, especially if the amount of moisture is small. 

 If the amount is large, however, 95 per cent, sulphuric acid is 

 as good as aluminium oxide, whilst calcium chloride is inferior 

 to both. 



ORGANIC CHEMISTRY. By P. Haas, D.Sc, Ph.D., St. Mary's 

 Hospital Medical School. 



A method for distinguishing Tautomeric, Isomeric, and Poly- 

 meric from Polymorphic substances has recently been pub- 

 lished by Sidgwick {Trans. Chem. Soc. 191 5, 107, 672). Many 

 examples are known of substances which are capable of existing 

 in two mutually interconvertible modifications ; this condition 



