406 Intelligence and Miscellaneous Articles. 



means of silicate of lime and other soluble silicates that phosphoric 

 acid is set free from its compounds with iron and alumina, and this 

 view was confirmed by the following experiments : — 



A solution of silicate of lime, mixed with phosphate of alumina, 

 pure or containing an excess of alumina, is put into one of the 

 common apparatus for preparing aerated water. The liquid is then 

 saturated, in the ordinary manner, with carbonic acid, and the 

 whole is left to digest for twenty-four hours, shaking it from time to 

 time. If the liquid be then removed and filtered, tlie ordinary reac- 

 tions demonstrate the presence of a considerable quantity of phos- 

 phate of lime. 



In operating upon a soil instead of phosphate of alumina, taking 

 the precaution to boil it for fourteen or fifteen hours with an excess 

 of silicate of lime, the same result is obtained. 



This experiment cannot furnish a means of determining the 

 amount of phosphates in a soil ; it is qualitative, and by no means 

 quantitative ; it is simply a moment of passage, but it is of this 

 moment that plants appear to take advantage in order to appropriate 

 the phosphates they require. Fortunately this moment is pro- 

 longed as long as there are silicates in solution, and as this is the 

 case in all soils in which plants grow, the latter are always fur- 

 nished with a larger or smaller quantity of protophosphates. It is 

 a sort of change effected at the expense of the silicates, whilst the 

 iron and alumina fix the unappropriated phosphoric acid, and retain 

 it in the soil for the benefit of future generations of plants. 



After instancing some districts in France in which the presence or 

 absence of phosphates or silicates in the soil directly influences the 

 selection of manures, the author states that it is especially to the 

 silicate of lime that he attributes the property of decomposing the 

 sesquiphosphates. The silicate of magnesia would probably have 

 the same action ; the silicates of soda and potash act upon the com- 

 pounds formed by stable manure in the soil, so as to complicate the 

 phsenomenon and render conclusions more doubtful. — - Comptes 

 Rendus, February 1, 1858, p. 212. 



ON THE MECHANICAL EQUIVALENT OF HEAT. BY P. A. FAVRE. 

 In the third part of my researches on hydro-electric currents*, it 

 ■was not so much my object to determine the mechanical equivalent 

 of heat by means of a new and, as I believe, very direct method, as 

 to supply new proofs of the existence of a single force, which, 

 though its manifestations vary, is never subjected to such modifica- 

 tions as to cause us to question its fundamental identity. In this 

 way the equivalence between the various species of manifestations 

 may be expressed by a number derived from any one of them. 



Thus, in the researches mentioned, where my chief aim was to 

 study the correlation between chemical affinity and the other dyna- 

 mic manifestations of matter, I showed that the amount of affinity 

 may be expressed in kilogrammetresf. In another series of investi- 



* Comptes Rendus de I' Acad, de Scien., July 13, 1857. 

 t One kilogramme, through one metre. 



