220 



serves, that luxuriant shoots, which abound in sap, constantly turn 

 upwards, and endeavour to acquire a perpendicular direction ; but 

 that the feeble and more slender shoots grow in almost every direc- 

 tion, probably from their fibres being more dry, and their vessels less 

 amply supplied with sap, so that they are less affected by gravitation. 

 To the second objection, Mr. Knight answers, that the compression 

 of the radicle, as it penetrates the soil, obstructs the motion of the 

 sap, and occasions the generation of numerous lateral roots ; and as 

 their substance is less succulent than that of the radicle first emitted, 

 they are less obedient to gravitation, and consequently extend hori- 

 zontally in every direction. Respecting the tap-root of the oak, the 

 author says he has examined at least 20,000 trees of that species, 

 and never found one tree that possessed a tap-root ; he therefore 

 thinks he may be allowed to doubt the existence of such a root. 



A third Series of Experiments on an artificial Substance, which pos- 

 sesses the principal characteristic Properties of Tannin ; with some 

 Remarks on Coal. By Charles Hatchett, Esq. F.R.S. Read Ja- 

 nuary 16, 1806. [Phil. Trans. 1806, p. 109.] 



Mr. Hatchett, in his former communications on this subject, gave 

 some account of the effects produced by sulphuric acid upon turpen- 

 tine, resin, and camphor. He now states the results of a variety of 

 experiments made with that acid upon a great number of resins, 

 balsams, gum-resins, and gums ; from which it appears, that sul- 

 phuric acid almost immediately dissolved the resins, forming transpa- 

 rent brown solutions, which gradually became black ; that the solu- 

 tions of the balsams and of guaiacum were at first of a deep crimson 

 colour, slightly inclining to brown ; and that caoutchouc and elastic 

 bitumen were not dissolved, but, after a long digestion, were only 

 superficially carbonized. 



Turpentine, common resin, elemi, tucamahaca, mastic, copaiba, 

 copal, camphor, benzoin, the balsams of Tolu and of Peru, assafoe- 

 tida, and amber, yielded a large proportion of the tanning substance ; 

 so also did oil of turpentine. 



Asphaltum yielded only a small portion of that substance ; and 

 some slight traces of it were obtained from gum arabic and from 

 gum tragacanth ; but none was produced from guaiacum, dragon's 

 blood, myrrh, gum ammoniac, olibanum, gamboge, caoutchouc, 

 elastic bitumen, liquorice, and manna. Mr. Hatchett thinks, how- 

 ever, that some of these would have yielded it, had not the digestion 

 with nitric acid been too long continued. 



Olive oil was partly converted into the tanning substance ; so also 

 were linseed oil, wax, and animal fat. In the experiment with lin- 

 seed oil, a portion was left undissolved : this portion appeared to 

 retain many of the properties of an inspissated fat oil. In the ex- 

 periment made with wax, a white substance was obtained, which 

 was found to possess the properties of spermaceti. In that with 

 animal fat (in which the kidney- fat of veal was employed), a great 



