PLANT-LICE. 



PLASTER OF PARIS. 



and gardening. As in the preceding article I 

 have gone very fully into the particulars of 

 planting trees, I shall only add in this place 

 the following useful table, showing the number 

 of plants required for one acre of land, from 1 

 foot to 21 feet distance from plant to plant. 



Number. 

 602 

 538 

 482 

 436 

 361 

 302 



194 

 171 

 151 

 135 

 121 

 109 



PLANT-LICE. See APHIIHAKS. 



PLASHING. A mode of repairing or modi- 

 fying a hedge by bending down a portion of 

 the shoots, cutting them half through near the 

 ground, to render them more pliable, and twist- 

 ing them among the upright stems, so as to 

 render the whole effective as a fence, and at 

 the same time preserve all the branches alive. 

 For this purpose the branches to be plashed or 

 bent down must not be cut more than half 

 through, in order that a sufficient portion of 

 sap may rise up from the root to keep alive the 

 upper part of the branches. Where hedges 

 are properly formed and kept, they can very 

 seldom require to be plashed; but this mode 

 of treating a hedge is most valuable in the cases 

 of hedges abounding with hedge-row trees, 

 when from neglect, or from any other cause, 

 the hedge has become of irregular growth. 

 See HEDGES. 



PLASTER OF PARIS, or GYPSUM. One 

 of the common names of the sulphate of lime 

 or plaster stone, which is found abundantly 

 near Paris. When burnt and reduced to pow- 

 der, and then mixed with water, it forms a firm, 

 sonorous substance, admirably adapted for 

 forming models and casts. 



PLASTER OF PAHIS or GTPSUM, AS A MA- 

 NURE. It is useless to search in the works of 

 the early agricultural writers for any notice of 

 the employment of gypsum as a manure. It 

 is true that Virgil speaks of the value of a very 

 impure variety of it, when he is commending 

 the use of ashes to the Roman farmers. The 

 early inhabitants of Britain thus used it; the 

 farmers of Lombardy did the same ; but ages 

 elapsed before even chemists were able to dis- 

 tinguish this salt from limestone, or other cal- 

 careous matter. Its uses, in its simple state as 

 a manure, were first noticed, according to Kir- 

 wan, about the middle of the 18th century, by 

 a very able German clergyman, of the name of 

 Meyer, who tried with success various expe- 

 riments with a mineral substance found in his 

 neighbourhood, which was long afterwards 

 shown to be an impure sulphate of lime. The 

 name of plaster of Paris, by which this sub- 

 stance is commonly known, arose from its 

 abounding in the neighbourhood of that capi- 

 tal, where it was burnt into a powder, and 

 used as a stucco. The composition of sulphate 

 of lime, when pure, is 

 113 



Sulphuric acid - 



Lime - 



Water - - 



Parti. 

 43 

 33 

 24 



100 



But the gypsum of commerce is usually united 

 with a portion of silica and carbonate of lim<?. 

 It is thus combined in its native state. Ac- 

 cording to Chaptal and Buchholz, gypsum con- 

 sists of 



Sulphuric acid 

 Lime - 

 Water - 



Parts. 



- 32 or 43 



- 30 or 33 



- 38 or 24 



There is, perhaps, no artificial manure so 

 decided in its effects upon some soils, so readily 

 obtainable by the farmer, and so plentiful, as 

 gypsum. Its mode of action, too, is now easily 

 understood. It acts as a direct food for some 

 plants. There are five commonly cultivated 

 crops which contain gypsum in sensible pro- 

 portions, and to which, in consequence, it is a 

 direct food, viz., lucern, sanfoin, red clover, 

 rye-grass, and turnips. Now, these are pre- 

 cisely the crops to which the farmer finds, on 

 most soils, gypsum to be a fertilizing top- 

 dressing. Wheat, barley, oats, beans, and 

 peas, do not contain a trace of this salt ; and 

 the farmer tells you that gypsum is of little or 

 no service to these crops, however the appli- 

 cation may be varied. That it does not ope- 

 rate by its attraction for atmospheric moisture, 

 I some time since determined by my own ex- 

 periments; for 1000 parts, previously dried, 

 when exposed to air saturated with moisture 

 for 3 hours, only gained 9 parts, while under 

 the same circumstances a good arable soil, 

 worth 2 guineas per acre, gained 14 parts; 

 and when compared with other manures, the 

 disproportion is still greater: thus soot gained 

 36 parts, and horse-dung 145 parts. That it is 

 not a promoter of putrefaction, I have ascer- 

 tained by mixing this salt with various animal 

 and vegetable substances ; it seemed, in every 

 case, rather to retard than promote the spon- 

 taneous decomposition of them all. The house- 

 wives consider hard water, which commonly 

 owes its properties to the presence of this salt, 

 to be a greater sweetener of tainted food than 

 soft water. Davy, also, in some experiments 

 with minced veal, thought that the addition of 

 the gypsum rather retarded putrefaction. 



There is no reason to believe that the pro- 

 portion of sulphate of lime found in certain 

 plants is as essential to their growth as the 

 presence of the other earthy salts and pure 

 earths. Thus, those plants which yield this 

 salt never grow well on lands which do not 

 contain it; those in which carbonate of lime is 

 found never flourish in soils from which this 

 salt is absent. Plants which abound with ni- 

 trate of potash (saltpetre), such as the sun- 

 flower and the nettle, always languish in soils 

 free from that salt ; but when watered with a 

 weak solution of it, their growth is very mate- 

 : rially promoted, and the saltpetre is then found 

 | in them, as shown, upon analysis, in very sen- 

 ' sible proportions. The same remarks apply 

 to the growth of those plants which contain 

 ! common salt, or phosphate of lime ; the effect 

 j is the same, the result invariable. 



897 



