434 



DAHLIA DALLAS. 



that exposure for a tenth of u second is more than 

 sufficient to make an impression on it. 



At the bottom of the dark chamber, which .M. 

 Daguerre has reduced to small dimensions, is a 

 plate of ground glass, which advances or recedes 

 until the image of the object to be represented is 

 perfectly clear and distinct. When this is gained, 

 the prepared plate is substituted for the ground 

 glass, and receives the impression of the object. 

 The effect is produced in a very short time. When 

 tin- metallic plate is withdrawn, the impression is 

 hardly to be seen, the action of a second vapour 

 being necessary to bring it out distinctly; the 

 vapour of mercury is employed for this purpose. It 

 is remarkable, that the metallic plate, to be pro- 

 perly acted upon by the mercurial vapour, must be 

 placed at a certain angle. To this end, it is en- 

 closed in a third box, at the bottom of which is 

 placed a small dish rilled with mercury. It the 

 picture is to be viewed in a vertical position, as is 

 usually the case with engravings, it must receive 

 the vapour of mercury at an angle of about 45. 

 If, on the contrary, it is to be viewed at that angle, 

 the plate must be arianged in the box in a horizon- 

 tal position. The volatilization of the mercury 

 must be assisted by a temperature of 60 of Reau- 

 mur (or 167 of Fahrenheit.) After these three 

 operations, for the completion of the process, the 

 plate must be plunged into a solution of hypo-sul- 

 phite of soda. This solution acts most strongly 

 on the parts which have been uninfluenced by light ; 

 the reverse of the mercurial vapour, which attacks 

 exclusively that portion which has been acted on 

 by the rays of light. From this it might perhaps 

 be imagined, that the lights are formed by the 

 amalgamation of the silver with mercury, and the 

 shadows by the sulphuret of silver formed by the 

 hypo-sulphite. M. Arago, however, formally de- 

 clared the positive inability of the combined wis- 

 dom of physical, chemical, and optical science, to 

 offer any theory of these delicate and complicated 

 operations, which might be even tolerably rational 

 and satisfactory. The picture now produced is 

 washed in distilled water, to give it that stability 

 which is necessary to its bearing exposure to light 

 without undergoing any further change. The art 

 of fixing the colours of objects has not hitherto 

 been accomplished ; and another important desid- 

 eratum is, the means of rendering the picture unal- 

 terable by friction. The substance of the pictures 

 executed by the Daguerrotype is, in fact, so little 

 solid is so slightly deposited on the surface of the 

 metallic plate, that the least friction destroys it, 

 like a drawing in chalk : and at present, it is ne- 

 cessary to cover it with glass. 



DAHLIA, (a.) The Dahlia, which now forms 

 so prominent a feature in the flower-garden, was 

 named in honour of Andrew Dahl, a botanist of 

 Sweden. Wildenow objected to the term, under 

 an erroneous impression that it had previously been 

 appropriated to another genus ; and adopted the 

 name Georgina ; but he has not been followed by 

 subsequent writers. Others objected to it from 

 its similarity to Dalea, a genus already established 

 after our countryman Dale. The name Dahlia is 

 now, however, so well confirmed, that it may bid 

 defiance to the caprice of modern botanical name- 

 changers. It is, notwithstanding, very desirable 

 that attention be paid to the proper pronuncia- 

 tion of. the word. The a should have the open 

 sound, as in father ; it will then be clearly distin- 

 guishable from the older name Dalea. The genus 



is now piincipally dividrd into two species, SHJXT- 

 flua and frustranea, in allusion to the florets of the 

 rays of the former abounding in seed, whilst those 

 of the latter species are barren. Other specific 

 distinctions were first adopted, but they all proved 

 unstable ; and from the proneness of the Dahlia to 

 sport into such numerous varieties, it may be 

 doubted whether the present distinction will prove 

 permanent. 



These splendid plants are natives of Mexico, and 

 though noticed by the Spaniards about the middle 

 of the seventeenth century, did not attract much 

 attention till they had flowered at Madrid, in IT'.Kt, 

 when Cavanilles described them in the first volume 

 of his Icones, published in the following year. In 

 1802, he sent plants to Paris, where they were 

 successfully cultivated by Monsieur Thouin, who, 

 shortly afterwards, published coloured figures and 

 a description of them. The first introduction of 

 the Dahlia into England was, according to the 

 Hortus Kewensis, by the marchioness of Bute, in 

 1789, but the plants, it may be presumed, were 

 soon lost. In 1802 and 1803, others were sent 

 from Paris; and in 1804, seeds from Madrid; yet, 

 for several years, they were scarcely heard of 

 amongst us. Their habits being unknown, their 

 increase was slow; whilst, on the continent, innu- 

 merable and splendid varieties were produced ; so 

 that, after the peace, in 1814, they were poured 

 upon us in all the variety of their present tints ; ex- 

 citing the astonishment of every beholder, and the 

 joy of those who could number such beauties 

 amongst their own collections. Since that time 

 they have been rapidly increased and improved ; and 

 England can now boast of varieties as superb as any 

 in the world. 



Early sown seeds produce plants that will flower 

 in the succeeding autumn. The more certainly if 

 forced on a hot-bed. Roots keep very well in 

 sand, in a dry cellar. In dividing them, the old 

 stems may be slit, and a portion must be retained 

 to each plant. Plant old roots in the first week of 

 April; or pot them, force in a hot-bed, and turn 

 into the borders when three or four inches high. 

 A few may be retained in large pots ; they will be 

 less luxuriant, and flower earlier. Train one stem 

 only from each root, and pinch off the lower-side 

 shoots. The superfluous shoots from old roots, 

 when taken off, may be planted in the shade, under 

 a hand-glass, and will readily grow, as will cut- 

 tings of the older stems. Or cuttings of fine va- 

 rieties may be grafted on the tubers of common 

 ones, merely by splicing them together, tying, and 

 enclosing them in a little clay, before they are 

 potted in mould : they should then be put in a hot- 

 bed and shaded. A gravelly soil checks their lux- 

 uriance and produces most flowers. 



DALLAS, SIB GEORGE, Bart., was descended 

 from an ancient family long established at Cantra 

 in Scotland, and was born in London, on the 6th 

 of April, 1758. He was educated principally at 

 Geneva, under the care of Mr Chauvet, a distin- 

 guished minister of the Swiss Church, together 

 with his brother, the late right honourable Sir 

 Robert Dallas, who afterwards becime a leading 

 ornament of the English bar, and lord chief justice 

 of the court of common pleas. At the age of 

 eighteen, Sir George (then Mr) Dallas sailed for 

 Bengal, as a writer in the service of the East In- 

 dia Company; and, having given proof of the most 

 promising abilities, was, soon after his arrival in 

 India, appointed to a station of some importance at 



