MAMUEING IN THEORY AND PRACTICE. 



37 



soientiflo system dictated by his knowledge of 

 chemistry, is said to have in nine years more than 

 doubled the produce of his fields. 



If, however, as is likely enough, a man \vith a 

 good knowledge of chemistry proved to he an un- 

 successful gardener, his failure would he due to the 

 fact of his heing unpossessed of a practical know- 

 ledge of horticulture, and not because he made him- 

 self acquainted with the principles of chemical 

 science ; indeed, we have in the above-named case of 

 Lavoisier, and of many others we might mention, 

 good reasons for supposing that a man who had 

 acquired a knowledge of the ait would be. a more 

 successful gardener than a person knowing as httle 

 of practical horticultme, and ignorant altogether of 

 chemical science. 



If, then, the results of investigations should tend 

 to explain and enforce good old practices, rather 

 than to put forth those which are new and untried, 

 the utility or even the necessity of the application of 

 science to the improvement of horticulture wUl not 

 be less evident 



Of What do Plsunts Consist ?— From what 

 we now know of the component parts, and of the 

 sources of the constituents of plants, it is obvious 

 that a knowledge of the composition of the atmo- 

 sphere, of water, and of the soil, is essential to any 

 right conception of the main features of vegetable 

 economy; and it is of interest to observe that in 

 the year 1610 a student of chemistry, named Van 

 Heboaont, asserted what he believed he had proved 

 by reliable experiment, namelj-, that vegetable life 

 derived its means of support entirely from water. 

 But although water is indispensable to vegetation, 

 from the fact that it supplies a medium for dissolving 

 all those nutritive substances which cannot of them- 

 selves become fluid, and because, moreover, its fluid 

 constitution is the means of the foi:mation of the 

 solid vegetable structure (for it is from the juices 

 made liquid by the water that all the solid con- 

 stituents of plants are built up), yet it was not until 

 the distinguished Henry Cavendish made his dis- 

 covery of the composition of water that the true use 

 and function of that element in the vegetable world 

 was understood. It is to the collective labours of 

 Black, Scheele, Priestley, Lavoisier, Cavendish, and 

 Watt, we owe the knowledge that common air 

 consists chiefly of nitrogen and oxygen, with a 

 little carbonic acid ; that carbonic acid is composed 

 of carbon and oxygen; and that water is composed 

 of hydrogen and oxygen; whilst Priestley and 

 Ingenhousz, Sennebier and Woodhouse, ravestigated 

 the mutual relations of these several bodies and 

 vegetable growth. 



Chemical analysis shows that fourteen elements 



enter into the composition of plants, which may be 

 divided into combustible (volatile) and memnbustible 

 (fixed) constituents. When a vegetable substance is 

 burnt — as, for example, tobacco — the greater part of 

 it is dissipated, becomes volatile ; but there remains 

 a white ash. This ash is the incombustible or fixed 

 mineral portion of the plant bm-nt, and is found on 

 analysis to contain chiefly the following constituents 

 — potassium, magnesium, calcium, iron, phosphorus, 

 sodium, silicon, sulphur, chlorine, and frequently 

 manganese. The first five elements, although they 

 may form but a comparatively small proportion of 

 the whole plant, are nevertheless indispensable to its 

 very existence. 



How greatly these incombustible or " mineral " 

 constituents may vary, not only in different plants, 

 but in one and the same plant when different organs 

 of that plant are taken, may be learnt from the 

 following table : — 



Percentage of Ash in 100 of Dry Svbstajice, 

 rs Different Plants. 



Ashes of Trees in 100 of Dry Sibstance. 



In looking at the preceding table, we cannot fail 

 to be struck -with one or two points, which are iUus- 

 tiated very remarkably : — 



[a] That the quantity of mineral matter con- 

 tained in the same weight of the different plants we 

 cultivate is most unlike. 



(J) The different quantity of ash yielded by 

 different portions of the same plant is equally 

 significant. Thus, 100 lbs. of the stems and leaves 



