166 WHAT DOES MAN LIVE UPON? 



vegetable mould. If we burn a plant, be it what it may, 

 we obtain a residue which does not become dissipated with 

 the products of combustion a variable quantity of ash. 

 Lime, silex, soda and potash, common salt and a mixture 

 of carbonate and phosphate of lime (called bone-earth, 

 because it forms the incombustible portion of bones), gyp- 

 sum and some other constituents, are the substances of 

 which the ash is usually composed. When we compare 

 among themselves, the results of the investigation of the 

 asbes of a large series of plants, we arrive at some remark- 

 able laws. We find that any given plant always yields 

 very nearly the same relative quantity of ash, that this ash, 

 within certain very narrow limits, defined by chemical prin- 

 ciples, has a regular composition. We discover, lastly, 

 that different plants leave behind after combustion, ashes 

 composed either of very different substances, or of very 

 different mixtures of these substances. 



Unreasonable as it would be to maintain that Arrow-root 

 only forms so pure a starch, in order that we may make 

 use of it for the food of our children and invalids, and that 

 this substance has no definite importance to the life of the 

 plant itself, it would be equally preposterous to assume that 

 plants only take up a perfectly definite quantity of the 

 constituents of the ash from the soil, in order that we may 

 here and there obtain potash from them, or that this ash 

 should be a troublesome residue in our stoves. We must 

 rather, from the phenomenon that certain plants regularly 

 take up certain inorganic, mineral constituents from the 

 soil, be led to the opinion that these constituents are as 

 essential to the existence, and consequently to the nutrition, 

 of the plant as those elements out of which it composes its 

 organic structures. It is quite a matter of indifference here 

 whether we are enabled by the condition of our science to 



