1820.] Physical Science during the Year 1819. 91 



several of them have of being converted into another principle 

 possessed of quite different properties. Thus starch, when 

 Lated with sulphuric acid, is changed into sugar Gum, when 

 treated with nitric acid, is converted into saclactic acid Ihis 

 property will doubtless hereafter throw much hght both upon 

 the constitution of vegetable substances, and upori the pheno- 

 mena of vegetation, which are at present wrapped up m great 

 obscurity. A set of experiments recently made by M. Bracon- 

 not has Idded several unexpected facts to the stock ot our know- 

 ledge respecting the changes of vegetable principles still so 



'""^hen^the sawings of the hornbeam weU dried are mixed and 

 ao-itated with sulphuric acid, considerable heat is evolved sul- 

 phurous acid is disengaged, and the whole becomes black. 

 %hen mixed with water, a black powder falls, which, when 

 dried, burns with flame, and possesses nearly the characters ot 

 sawino-s of hornbeam exposed to the air and moisture for 

 several years. The sulphurous liquor was nearly colourless, and 

 being saturated by carbonate of lime, and filtered, yielded, when 

 evaporated, a gum possessing many characters similar to those 



of Efum arabic. ,. ■ j • i 



flemp cloth, being treated in the same way, dissolved m sul- 

 phuric acid, but no sulphurous acid was disengaged Ihe 

 solution, being mixed with water, and treated as that of the saw- 

 ings of hornbeam, yielded a quantity of gum quite similar in its 

 characters to the ^um from the hornbeam : 21-5 parts of hemp 

 furnished 26-2 parts of this gum, which Braconnot found com- 

 posed as follows : 



Matter from the hemp 21-50 



Elements of sulphuric acid fixed in an 



unknown manner -^ oo 



Elements of water, ditto 0*40 



Lime 1-47 



26-20 



The gum is transparent, yellowish, without smell or taste, and 

 breaks with a vitreous fracture. However, it reddens vegetable 

 blues. It adheres strongly to bodies, and forms a very shining 

 varnish. Its solurion in water has less tenacity than that ot gum 

 arabic. When heated sufficiently, it burns, giving out a strong 

 odour of sulphurous acid. There remains a charcoal, which, 

 after incineration, leaves some traces of sulphate of hme. Us 

 solution in water is not precipitated by nitrate ot barytes, or 

 acetate of lead ; but subtritacetate throws down a copious preci- 

 pitace which is totally dissolved by acetic a(-id. , . . . , 

 If this gum be boiled for some time with dilute sulphuric acid, 

 it is converted into two new substances ; namely, s«gar, and an 

 *cid which Braconnot has called vegeto-su/phunc acid. Iwenty- 



