CHEMISTRY. 



127 



tiun of 

 Kature. 



Chemical If oxymuriatic acid be poured into the milky juice, 

 tne caoutchouc precipitates immediately, and at the same 

 time the acid loses its peculiar odour. This renders it pro- 

 bable that the formation of the caoutchouc is owing to 

 its basis absorbing oxygen. If the milky juice be confi- 

 ned in a glass vessel containing common air, it gradually 

 absorbs oxygen, and a pellicle of caoutchouc appears on 

 its surface. 



Caoutchouc, when pure, is of a white colour, and with- 

 out either taste or smell. The blackish colour of the 

 caoutchouc of commerce is owing to the method etn- 

 ployd in drying it after it has been spread upon moulds. 

 The usual way is to spread a thin coat of the milky juice 

 upon the mould, and then to dry it by exposing it to 

 smoke ; afterwards another coat i* spread on, which is 

 dried in the same way. Thus the caoutchouc of com- 

 rr>.erce.consi8ts of numerous layers of pure caoutchouc al- 

 ternating with as many layers of soot. 



Caoutchouc is soft and pliable like leather. It is ex- 

 ceedingly elastic and adhesive, so that it may be forcibly 

 tretched out much beyond its usual length, and instant- 

 ly recover its former bulk when the force is withdrawn. 

 It cannot be broken without very considerable force. 

 Its specific gravity is 0.9335. 



Caoutchouc is not altered by exposure to the air ; it 

 is perfectly insoluble in water ; but if boiled for some 

 time, its edges become somewhat transparent, owing un- 

 doubtedly to the water carrying off the soot ; and so 

 toft, that when two of them are pressed and kept toge- 

 ther for some time, they adhere as closely as they lorm- 

 ed one piece. By this contrivance pieces of caoutchouc 

 may be soldered together, and thus made to assume 

 whatever shape we please. 



Caoutchouc is insoluble in alcohol. This property 

 was discovered very early, and fully confirmed by the 

 experiments of Mr Macquer. The alcohol, however, 

 renders it colourless. 



Cacutchotic is soluble in ether. This property was 

 first pointed out by Macquer. Berniard, on the con- 

 trary, found that caoutchouc was scarcely soluble at all 

 in Milphuric ether, which was the ether used by Mac- 

 quer, and that even nitric ether was but an imperfect 

 solvent. The differ- nee in the results of these two che- 

 muts was very singular ; both were remarkable for their 

 accuracy, and both wen- too well acquainted with the 

 subject to be easily muled. The matter wag first cleared 

 up by Mr Cavallo. He found that ether, when newly 

 prepared, seldom or never dissolved caoutchouc complete- 

 ly ; but if the precaution was taken to wash the ether 

 pn-viously in water, it afterwards dissolved caoutchouc 

 with t.tcility. 



W.ien the ether is evaporated, the caoutchouc is ob- 

 tained unaltered. Caoutchouc, therefore, dissolved in 

 ether, may be employed to make instruments of different 

 kinds, just as the milky juice of the heevea ; but this 

 method would be a great deal too expensive for common 

 ile. 



Caoutchouc is soluble in volatile oils ; but, in general, 

 when these oils are evaporated, it remains somewhat glu- 

 tinous, and therefore is scarcely proper for those uses to 

 which, before its solution, it was so admirably adapted. 

 The acids act but feebly upon caoutchouc. Sulphu- 

 ric acid, even after a long digestion, only chars it super- 

 ficially. When treated with nitric acid, there came over 

 azotic gas, carbonic acid gai, pru sic acid gas ; and ox- 

 alic acid is said to be formed. Muriatic acid does not 

 affect it. The other acids have not been tned. 



Fabroni has discovered, that rectified petroleum dis- 

 *ol<rei it, and leaves it unaltered when evaporated. 



When exposed to heat it readily melts ; but it never Chemical 

 afterwards recovers its properties, but continues always Ejcamina- 

 of the consistence of tar. It bunts very readily with a 

 bright flame, and diffuses a fetid od"ur. In those coun- 

 tries where it is produced, it is often used by way of candle. 



When distilled it gives cut ammonia. It is evident 

 from this, and from the effect of sulphuric and nitric 

 acid upon it, that it is composed of carbon, hydrogen, 

 azote, and oxygen ; but the manner in which they are 

 combined is unknown. 



SECT. XXVII. Of Gum Resins. 



This class of vegetable substances has been long dis- Gum re- 

 tingui-ihed by physicians and apothecaries. It contains slnSl 

 many active substances much employed in medicine ; and 

 they certainly possess a sufficient number of peculiar 

 properties to entitle them to be ranked apart. Unfor- 

 tunately these substances have not yet attracted much 

 of the attention of chemists. Their properties and eon- 

 stituents, of course, are but imperfectly ascertained. 



They are usually opake, or at least their transpa- 

 rency is inferior to that of the resins. They are always 

 solid, and mi st commonly brittle, and have sometjmet 

 a fatty appearance. 



When heated, they do not melt as the resins do ; nei- 

 ther are they go combustible. Heat, however, com- 

 monly softens them, and causes them to swell. They 

 burn with a flame. 



They have almost always a strong smell, which in se- 

 veral instances is alliaceous. Their taste also is often 

 acrid, and always much stronger than that of the resins. 



They are partially soluble in water ; but the solution 

 is always opake, and usually milky. 



Alcohol dissolves only a portion of them. The solu- 

 tion is transparent ; but when diluted with water it 

 bee. mes milky; yet no precipitate falls, nor is any thing 

 obtained by filtering the solution. 



Vinegar and wine likewise dissolve them partially ; and 

 the solution, like the aqueous, is opaque or milky. 



According to Hermbstadt, they are insoluble in sul- 

 phuric ether. 



The action of alkalies on them has been examined 

 only by Mr Hatchett. All of them tried by that cele- 

 brated chemist, dissolved readily in alkaline solutions, 

 when assisted by heat. We may therefore consider them 

 as soluble in alkalies like resins. 



Nitric acid acts upon them with energy; converting 

 them first into a brittle mass, and then, with the assist- 

 ance . f heat, dissolving them. 



Their specific gravity is usually greater than that of 

 the resins. 



TlieT other properties still continue unknown. They 

 all either exude spontaneously from plants, or are ob- 

 tained by incisions. At first ihey seem to be in a liquid 

 state ; but they gradually harden when exposed to the 

 air and weather. 



The gum resins which have been hitherto applied to 

 any useful purpose are the following : 



1. Galbmium. It is obtained from the bubon galba- Galbanum 

 HUM, a perenmal plant, and a native of Africa. When 

 this plant is cut across a little above the root, a milky 

 juice flows out, which soon hardens and constitutes gal- 

 banum. It comes to this country from the Levant, in 

 small pieces, composed of tears, agglutinated together, 

 of a yellowish or white col >ur. Its taste is acrid and 

 bitter, and its smell peculiar. Water, vinegar, and wine, 

 dissolve part of it, but the solution is milky. Alcohdl 

 dissolves about three-filths. 



