306 Col. Sir H. James and Capt. A. R. Clarke on P?*ojections 



soluble in the bichloride of the same metal. Such a solution, 

 when placed in the dialyser, allows the whole chlorine of the 

 salt and a portion of the tin to diffuse away. Peroxide of tin, 

 or stannic acid, remains behind, but not in a soluble state. It 

 forms in the dialyser a semitransparent gelatinous cake, which 

 after a few days is entirely free from chlorine. The original 

 solution, containing excess of stannic acid, was diluted to various 

 degrees, but was dialysed always with the same result. The 

 coagulum was insoluble in hot or cold water, but dissolved 

 readily in dilute acids. It was evidently the peroxide of tin 

 unaltered. 



The metastannic acid, or nitric-acid peroxide of tin of Berzelius, 

 forms a solid compound with a small quantity of hydrochloric 

 acid. This compound is not dissolved by an excess of acid, but 

 is soluble in pure water. The solution placed in the dialyser is 

 readily decomposed, and leaves behind a semitransparent gela- 

 tinous mass of pure hydrated metastannic acid, insoluble both 

 in water and acids. There appears, then, to be no soluble form 

 of either hydrated stannic or metastannic acid, although both 

 are colloidal substances. 



Precipitated titanic acid was dissolved in hydrochloric acid and 

 submitted to dialysis. The hydrochloric acid readily diffused 

 away, leaving hydrated titanic acid, gelatinous and insoluble, 

 upon the dialyser. The proportion of titanic acid which escaped 

 from the dialyser and was lost amounted to 0*050 gramme out 

 of 2*5 grammes. Titanic acid thus resembles stannic acid in 

 not presenting itself in the form of a fluid colloid. 



Metallic protoxides are not soluble in their neutral salts, and 

 therefore cannot be submitted to dialysis in the same conditions 

 as the preceding peroxides. It was observed, however, that 

 oxide of copper and oxide of zinc, when dissolved in ammonia, 

 are capable of diffusing through a colloidal septum, and are 

 therefore not colloids themselves. The water outside the dia- 

 lyser should be charged with ammonia in such an experiment. 

 [To be continued.] 



XLII. On Projections for Maps applying to a very large extent 

 of the Earth's Surface. By Colonel Sir Henry James, R.E., 

 Director of the Ordnance Survey ; and Captain Alexander R. 

 Clarke, R.E.* 



[With a Plate.] 



IN reading the "Explanation of a Projection by Balance of 

 Errors for Maps applying to a very large extent of the 

 Earth's Surface, and comparison of this projection with other 

 * Communicated by the Authors. 



