378 M. Weltzien on the Determination of Nitric Acid. 



Silico-tungstic acid crystallizes at ordinary temperature in 

 thick quadratic pyramids, and which contain 29 equivs. water of 

 crystallization, according to the formula 



Si0 2 12W0 3 4HO + 29Aq: 



if it crystallizes at somewhat higher temperature, or at ordinary 

 temperature in the presence of alcohol, hydrochloric acid, or sul- 

 phuric acid, it forms a hydrate which only contains 18 equivs. 

 water of crystallization, and of a different crystalline form. At 

 100° the acid retains, besides the baaic water, 4 equivs. of water 

 of crystallization. On heating at 220° it loses the latter, but 

 retains 2 equivs. of basic water : these appear necessary for the 

 constitution of the acid; they are only removed at the temperature 

 of red heat. ; . 



Tungsto-silicic acid crystallizes with 20 equivs. of water of 

 crystallization, and has the formula 



12 WO 3 , SiO 2 4HO + 20Aq. 



It is acted upon by heat in the same way as silico-tungstic acid. 

 Marignac has made a crystallographic investigation of the salts 

 of these acids; and although relations to other compounds have 

 not been found (since these acids appear to belong to a new type of 

 combination), they have furnished a number of interesting results. 

 These, as well as reasons against Persoz's proposal to change the 

 formula of tungstic acid, Marignac discusses at length. 



Weltzien* describes a method for the quantitative estimation 

 of nitric acid in waters. It consists in determining it as ni- 

 trogen gas. The water to be investigated is evaporated, and 

 by a careful addition of carbonate of soda, the lime and mag- 

 nesia are precipitated as carbonates, and any nitrate of lime or 

 magnesia is converted into nitrate of soda. The nitrate from 

 this precipitate is evaporated to dryness, and mixed with finely 

 divided copper reduced from the oxide by hydrogen. This mix- 

 ture is heated in a somewhat long combustion-tube in the usual 

 mode of determining nitrogen, all the atmospheric air having 

 been previously expelled by carbonic acid. Weltzien gives a 

 number of analyses of the waters of the town of Carlsruhe inves- 

 tigated by this method, and also, to show its accuracy, determi- 

 nations made with known quantities of nitrates. His analyses 

 of the town well-waters show in a marked degree the influence 

 of population in increasing the quantity of nitrates. They show 

 also the completeness with which any nitrogenous organic matter 

 becomes oxidized into nitrates. 



* Liebig's Annalen, November 1864. 



