of Insoluble Substances by Artificial Means. 181 



the table, readily examine much in a short time, and for a long 

 time, too, without the least fatigue. I conceive that this is by 

 much the most simple and useful microscope. 



As I am not near the termination of my labours on crystalli- 

 zation, I have thought it right, in the mean time, to publish a 

 few of the results I have obtained. I purpose shortly to pre- 

 sent to the Academy additional new crystals made in a similar 

 manner, and at the same time a microscope made on the prin- 

 ciples to which I have adverted. 



3. On the Operation of the Earths in the Process of Vegetation. 



The following is an abstract of a paper on this subject, lately 

 communicated to the French Academy of Sciences by M. Pelle- 

 tier. Resting his opinion on the observations of agriculturists 

 and chemists, and more particularly on the analyses of various 

 soils made by Chaptal, Davy, and himself, the author admits 

 that a fertile soil must be formed of silica, alumina, and lime ; 

 that the fertility diminishes as one of these three earths predo- 

 minates ; and that it is almost null when the mixture presents 

 only the properties of one of them. But why, and how is this 

 mixture of three earths, with the addition of oxide of iron or 

 magnesia, a necessary condition of fertility .'' " This question," 

 says M. Pelletier, " has not yet been satisfactorily answered. 

 The physical constitution of the soils, their hygroscopic pro- 

 perties, their power of being more or less strongly heated by 

 the solar rays, are "circumstances to which a certain influence 

 may reasonably be attributed, but which, nevertheless, appear 

 to be only secondary causes. It seems, on the contrary, evi- 

 dent, that the mixture of the different earths which compose 

 the soil, acts on vegetation by an electrc-chemical force, whose 

 influence has been recognised in other cii'cumstances, but which 

 has not hitherto been pointed out as connected with the subject 

 now under consideration." 



M. Pelletier remarks, that in a fertile vegetable soil the si- 

 lica, lime, and alumina, must exist in a state of simple mixture ; 

 that if these substances were combined the soil would be ste- 

 rile ; and that, in a mixture of these three earths, the fertility 

 would cease, if the combination were to be effected instantane- 

 ously. " For," says he, " in a mixture of silica, alumina, and 



