LEtTCIC ACID. 



LEVELLING. 



191 



ing and rendering them more crisp and delicate. This should be done 

 about a week previous to their attaining full perfection. The 

 Paris Cove Cos requires least assistance in this way, because the tops 

 of its leaves are concave, and successively apply themselves closely to 

 each other. To obtain a constant supply, successive sowings are 

 requisite to be made from February to July for the summer crop. 

 Those plants intended to stand the winter should be sown in the end 

 of August or beginning of September ; and when fit, should be trans- 

 planted to the bottom of walls or other fences having a south aspect, 

 or to the sides of slopes or ridges, made for the purpose, over which 

 a protection of mats, supported on hoops, may be formed. When the 

 demand is such as to require greater security, recourse must be had to 

 frames or pita. The first full supply from the open ground is best 

 obtained by sowing under glass on a decayed hot-bed in the second 

 week in October. When the plants come up they should be regularly 

 thinned. Abundance of air should be admitted when the state of the 

 weather will permit, and when mild the plants should be fully ex- 

 posed ; but the slightest frost should be guarded against. Very little 

 water will be required ; in fact the effects of damp are to be dreaded, 

 and therefore every favourable opportunity should be taken for the 

 admission of air, provided it is not saturated with moisture, unless 

 when too low a temperature would render such a proceeding inju- 

 dicious. A full exposure to air, both night and day, is necessary for 

 some time previous to planting out in the open ground, which 

 operation may take place in February, if the weather then prove favour- 

 able. A reserve should be kept in the frames in case of severe frost 

 occurring after the plantation has been made. 



LEUCIC ACID. [LEUCIN.] 



LEUCIN (C^H^NO,) Aposepidin. Leucin is a white crystalline 

 substance frequently met with in chemical operations upon animal 

 substances. Its composition and properties indicate that it is homo- 

 logous with sugar of gelatin [GLYCOCOI.L], and with ALANIN, as seen 

 from the following formuhc : 



GlycocoU 



Alanin 



Leacin 



C.H,NO, 



Gerhardt considered it as a hydrocyanic acid derivative of hydride 

 of valeryl, formed as follows : 



f, .'). 



Hydride of 

 Talcryl. 



+ C 2 HN 



Hydrocyanic 

 acid. 



Leucin. 



Limpricht has, in fact, within the last few years obtained leucin by 

 the action of hydrochloric and hydrocyanic acids upon valeride of 

 ammonium. 



Leucin is a product of the putrefaction of cheese and of gluten in 

 the presence of water ; also of the action of sulphuric acid, or hydrate 

 of potash upon muscular flesh, wool, white of egg, gelatin, horn, &c. 



The preparation of pure leucin is a somewhat tedious operation. 

 The simplest process is that adopted by Mulder ; it consists in boiling 

 albumen, gelatin, or fibrin, with caustic potash until decomposition is 

 fviduntly complete, evaporating the liquor to dryness, extracting the 

 residue with alcohol, and evaporating the filtered liquid, that the leucin 

 may crystallise out. It is finally purified by repeated recrystallisatiou 

 from alcohol. 



Pure leucin occurs in soft, light, colourless plates, somewhat re- 

 sembling cholesterin. It is slightly soluble in cold but readily so in 

 hot water. In alcohol it is not very soluble, and in ether it is insoluble. 

 Heated to 338 Fahr., it sublimes without melting and without decom- 

 position, and condenses in beautiful silky flocks. Fused with hydrate 

 of potash, it is decomposed into valeric acid, ammonia and hydrogen 

 being evolved. 



One equivalent of hydrogen in leucin may be replaced by cdpper, 

 mercury, &c., and compounds formed soluble in water and deposited 

 on evaporation in a granular or lamellar form. The copper compound 

 contains (C 1I H,,Cu NO,) ; the mercuric combination (C,,H 11 Ug NO,). 



Leucin is easily dissolved by the dilute acids, combining with them 

 to form bodies that crystallise out on evaporation. The nitrate of 

 leucin, sometimes called nitrolcucic acid, has the formula (C^H^NO,, 

 HO NO,) ; it crystallises in colourless needles. The hydroclilorate and 

 the ndphate are also crystalline. 



Leucic acid. When binoxide of nitrogen ia passed into a solution of 

 leucin ill nitric acid, nitrogen is evolved and an oily body obtained, to 



which the name of leucic acid has been given. It is very soluble in 

 ether, gives crystallisable salts with bases, and is homologous with 

 lactic acid. 



LEUCOL. [LEUCOLIXE.] 



LEUCOLINE, Leucal, Quinoline (C 18 H,N), is orie of the compounds 

 found in the least volatile portions of the basic oil of coal-tar. It is 

 also formed when quinine, cinchonine, or strychnine are heated iu 

 contact with potash. It is a liquid with a disagreeable smell, and 

 boiling at the temperature of 460. It neutralises acids, aud forms 

 with them salts. 



Greville Williams has recently shown that the substance to which 

 the names leucoline and quiuoline have been given is a mixture of two 

 homologous bases namely, quinoline (C 1S H,N), and lepidine (C M H 9 N). 



LEUCO'MA, a white opacity of the cornea. [EYE: NAT. HIST. 

 Drv.] It is the result of acute inflammation producing a deposition of 

 lymph on the surface and in the layers of the cornea, either with or 

 without ulceration of its substance. In those cases in which there is 

 merely an effusion of lymph on the surface, or between the superficial 

 layers of the membrane, it is often re-absorbed on the cessation of the 

 inflammation, and the cornea recovers its transparency. But when the 

 disease is more extensive and more deeply seated, the probability 

 of recovery is far less, and many such cases are incurable by any 

 means at present known. The most efficient mode of treatment is 

 that with astringent lotions, such as a solution of nitrate of silver, 

 in the proportion of from one to five grains to the ounce of dis- 

 tilled water. 



LEUCORCEIN. [LICHENS, COLOURING MATTER OF.] 



LEUCOTURIC ACID. [URIC ACID.] 



LEVELLING is the art of determining the heights or depressions 

 of points on the ground with respect to a spherical or spheroidal surface 

 coinciding nearly with that of the earth, or, when the extent of ground 

 is inconsiderable, with respect to a horizontal plane passing through 

 some given point on the ground. 



In extensive operations of this nature, which are connected witii 

 the researches of physical astronomy, an attention to that figure of the 

 earth which approaches the nearest to the truth is of importance ; but 

 when the object is merely to determine the profile of the ground, for a 

 canal, or a line of road, it is sufficient to consider the surface to which 

 the points are referred as that of a sphere. 



The relative' heights of a series of points on the ground are obtained 

 by means of their vertical distances from others which, on the supposi- 

 tion of the earth being a sphere, are equally distant from its centre ; 

 and these, which are called level-points, must be found by an instru- 

 ment constructed for the purpose. Now, a plane being supposed to 

 touch the earth at any given point, all the points in the circumference 

 of a circle described on that plane, about that point of contract as a 

 centre, will be level-points : consequently, if a telescope be so adjusted 

 that, when turned round upon the vertical axis of the instrument to 

 which it is applied, its line of collimation (that which passes through 

 the centres of all the lenses) may remain parallel to the horizon, any 

 number of such level-points will be determined, if, being at equal dis- 

 tances from the said axis, they are in the direction of the line of 

 collimation produced. The instrument alluded to is called a spirit- 

 level [SPIRIT LEVEL ; THEODOLITE] ; and by certain adjusting screws 

 the line of collimation, or optical axis of the telescope, is capable 

 of being brought into the position above mentioned, which is indi- 

 cated by a bubble of air remaining, during a complete revolution 

 of the telescope, in the middle of the tube containing the water or 

 spirit. 



The instrument is employed for the purpose of ascertaining the 

 relative heights of points on the ground in either of the following ways, 

 the first of which is the most simple, and is frequently adopted. 

 Choice is made of any convenient stations, A, c, &c., on the line of 

 operation, and the distances between them are determined either by 

 actual admeasurement, or by computations founded on the data 

 afforded by a previous survey of the ground. The instrument ia then 

 set up at, or near, the middle of the interval between every two such 

 points in succession. When the telescope thus placed, as at a, has 

 been rendered horizontal by means of the adjusting screws, an assistant 

 at each of the stations A aud B, holding what is called a station staff 

 in a vertical position, moves a vane or index along the staff, upwards 

 or downwards, according to the directions of the observer at the 

 telescope, till it appears to coincide with the intersection of two wires 

 in the telescope, that intersection having, by the adjustment of the 

 instrument, been made to coincide with the optical axis, or line of 

 eollimation. 



The points thus determined on the staves are represented by m and 

 n ; and, from what has been said, these are level-points, or points 

 equally distant from the centre of the earth. Therefore, the heights 

 A m and B being read on the graduated staves, the difference between 



ARTS AND 8CT. DIV. Vol.. V. 



them will give the relative heights of the ground at A and B ; that 

 point, of course, being the highest at which the distance of the vane 

 from the ground is the least. A similar process is repeated with respect 

 to the points B aud c, the instrument being placed at b, midway 



