120 MM. Bussy and Lecanu on the 



the dimensions of the earth, before they knew the method of 

 accurately determining the weight of a body. The whole 

 passage is very interesting : " Lorsqu'on enterprit, en France, 

 la determination des grandes unites de poids et des mesures, 

 on connaissait parfaitement le cours du ciel et les mouvemens 

 des astres ; on savait tres-bien mesurer les dimensions de la 

 terre ; mais on ne savait pas determiner exactement le poids 

 d'un corps, et il fallut que Borda inventat la methode des 

 doubles pesees pour y suppleer." 



Art. XXVII. — Note concerning the Presence of Anhydrous 

 Persulphate of Iron in the residue of the concentration of 

 Sulphuric Acid. By MM. Bussy and Lecanu. * 



In concentrating the sulphuric acid of commerce, prepared 

 in the usual way by the combustion of sulphur and nitrate of 

 potash, a white powder is gradually deposited, which the acid, 

 in its more diluted state, had held in solution. The deposit 

 has been hitherto regarded as a sulphate of lead ; but, on ex- 

 amining it chemically, MM. Bussy and Lecanu discovered that 

 it was a sulphate of the peroxide of iron, mixed occasionally 

 with a small quantity of silver. 



This observation led them to examine the action of strong 

 sulphuric acid upon the salts of iron. When the crystalliz- 

 ed protosulphate of iron (green vitriol) is put into the acid, 

 it quickly loses its green colour and becomes white, in con- 

 sequence of being deprived of its water of crystallization by 

 the concentrated acid. A part of the anhydrous salt sub- 

 sides as a white powder to the bottom ; but one portion of it 

 is at the same time dissolved by the strong acid, forming a 

 beautiful rose-coloured solution, which passes into purple as 

 more of the salt is dissolved. The green protosulphate of 

 iron, previously deprived of its water of crystallization, acts 

 in the same manner. 



The colour of the solution may be destroyed in two ways. 

 The first is by adding water to a certain extent, when the 



* Abstract from the Annates de Chimie, vol. xxx. 

 1] 



