336 Analysis of Scientific Books. 



and paper stained with their colours*, are also good tests, of acid 

 and alkali; but less sensible than the preceding. 



The ninth chapter of MM. Payen and Chevallier's work treats of 

 the application of different re-agents to analysis; here we find little 

 worthy of remark. Their instructions are neither so systematic nor 

 so precise as those given by M. Thenard. The method of detecting 

 magnesia, invented by Dr. VVollaston, and published long ago by 

 his lamented friend. Dr. Marcet, in the second edition of Saunders 

 on Mineral Waters, is briefly mentioned by our authors. The 

 elegant manner in which the inventor practises this process on the 

 smallest scale, having been somewhat vaguely described by 

 M. Clement, in the Annals de Chimie et de P/it/sique, for July, 

 1822, we shall take the liberty here of detailing it more precisely. 



Dissolve in a watch-glass, at a gentle heat, a minute fragment 

 of the mineral suspected to contain magnesia, dolomite for ex- 

 ample, in a few drops of dilute muriatic acid ; to this solution, add 

 oxalic acid, to render the lime that may be present insoluble; 

 then pour in a few drops of a solution of phosphate of ammoniaf . 

 Allow the precipitate to settle for a few seconds, and decant a 

 drop or two of the supernatant clear liquid on a slip of window- 

 glass ; on mixing with this liquid two or three drops of a solution 

 of the scentless carbonate of ammonia, an effervescence takes 

 places ; draw off to one side with a glass rod, a little of the clear 

 solution, and trace across it, with the pressure of a point of glass 

 or platina, any lines or letters on the glass plane ; on exposing this 

 to the gentlest possible heat (as making a little warm water flow over 

 it), white traces will be perceived wherever the point was applied. 

 These consist of the triple phosphate of ammonia and magnesia. 



The whole of this beautiful analytical operation, as performed 

 by Dr. Wollaston, occupies less time than we have taken to write 

 the formula. In the application of this process on the larger scale, 

 the carbonate of ammonia should be added first, which prevents 

 the chance of any simple phosphate of the earth being formed. 

 To estimate the quantity of magnesia present in any compound, 

 we must consider, according to Dr. Marcet, every 100 grains of 

 the triple salt dried at the temperature of 100° F. to be equivalent 

 to 19 of earthy base. If we calcine the salt at a red heat, so as 

 to expel all the water and ammonia, an earthy phosphate re- 

 mains, which, according to Dr. Murray, contains 40 per cent, of 

 magnesia. The theoretic proportions of the bi-phosphate of mag- 

 nesia are 70 acid -f 25 base, or 73.68 -|- 26.32 in 1 00 parts, and of the 

 neutral phosphate 35 acid-|-25 base, or 58^-f 41-j in 100 parts. 



Nitrate of mercury has been lately prescribed as a test for 

 examining sophisticated olive oil ; it is prepared by dissolving in 

 the cold 6 parts of mercury in 7^ parts of nitric acid, specific 



♦The blue petals of the iris or water-flag, aflFord a good test colour either 

 by infusion or by rubbing them on paper. Radish colour is transferred in 

 the latter way, and forms a pretty delicate test paper, 



t Phosphate of soda may likewise be used. 



