180 FIRESIDE SCIENCE. 



great variety of common and well known elements 

 and compounds, but also, in the case of many 

 plants, some of the most rare and curious of which 

 science has any knowledge. Potash, lime, nitro- 

 gen, phosphorus, and silica are almost universally 

 found in the sap and substance of plants, and these 

 are the elements which the farmer is desirous of 

 placing in the soil, so that the water may dissolve 

 and convey them in ample abundance to vegetable 

 structures. These are the great essentials, so far 

 as the supply is connected with the agency of 

 water, and they are in a measure accessible, but 

 there are other bodies of the highest importance 

 which it is impossible to furnish. We can supply 

 in a large measure the nitrogenous and other ele- 

 ments which are common to the cereal grains, but 

 we cannot in the case of many of the esculent veg- 

 etables. It is certainly remarkable that the com- 

 mon garden beet demands from the water of its 

 circulation one of the rarest of all minerals, rubid- 

 ium. This metal has only recently been made 

 known to us, through the agency of that marvellous 

 optical instrument, the spectroscope. When and 

 how the water finds and takes up this strange 

 metal, is a problem we are wholly unable to solve. 

 By spectroscopic analysis we are able to detect the 

 thirty thousandth part of a grain of the chloride of 

 the metal ; yet it is so sparsely disseminated that 



