Mechanical Science. '^ 331 



oxide of iron 32.75, carbonic acid 2.25. Here the lime was neu- 

 tralized not by carbonic acid, but by silica; and from circum- 

 stances M. Vicat thinks it probable that the silica in combination 

 was furnished by the terras. From the whole of the facts it seems 

 that terras, a substance consisting of hard absorbent particles, and 

 being a true puzzolana, cannot furnish carbonic acid to the lime, 

 and yet with the purest lime, even that of shells, it furnishes a 

 good hydraulic cement. 



In answer to other doubts of M. Berthier, who inquires why, if 

 common lime acts upon silica, it should not also act upon crude 

 clay, which enters so much more readily into combination than cal- 

 cined clay, and why also felspar sands act merely as pure siliceous 

 sands, M. Vicat quotes some new facts ; from which it appears 

 that substances not baked, not porous, not absorbent, and composed 

 of feeble elements, are competent to the neutralization of common 

 lime, so as to form with it hydraulic mortars. These substances 

 are 1. certain felspar sands or disintegrated granites ; 2. the 

 greater number of the friable bro^vn psammites of Bas Brittany. 

 Results have been obtained of this kind by MM. Avril and Payen, 

 and other experiments are in train for their more particular con- 

 firmation. — Ann. de Chimicy xxviii. 142, 



6. On the Cultivation of the Potato considered as to its produce in 

 potash and in roots. By jVI. J. B. Mollerat. — In consequence of 

 the contradictory accounts given by experimentalists as to the 

 produce of potash by the stalks and leaves of potatoes, M. Mollerat 

 undertook certain experiments on the subject. He found in 1818, 

 that the green crop gave its maocimum of potash immediately before 

 the flowering of the plant, and its minimum at the time of maturity. 

 At that time he determined to ascertain the effect of the leaves on 

 the roots, in order to ascertain whether there would be any ad- 

 vantage in collecting the alcali, but had not the opportunity of 

 making the experiment till 1824. A silico-argillaceous soil, rich 

 in alluvium and manure, was then planted ^vith the potato called 

 patraquejaune^ one of the most productive species, the cultivation 

 being carried on with care. 



The following table exhibits the produce in green crop, salts, 

 and sub-carbonate of potash, at the time of each cutting, and of 

 potatoes collected when the plant was mature ; the whole calcu- 

 lated on the results of 30 centiares* of ground, and multiplied to 

 represent the produce of a hectare t. 



♦ 1.2 square yards nearly. 



t 2 acres I rood 35.4 perches. 



NOTJB. — A kilogramme is 2 lb. 8 oz. 5 dr. 



