Experiments on Eatt de Luct. l6j 



however not fufficlcntly permanent. After a certain time — vliicli is longer in proportion to 

 the vceaknefs of the refinous fokition, and the flrength and caufticity of the ammoniac — this 

 aUb feparatcs, and the maftic forms a har.l adhelive coagulum which floats on tlie furface 

 of the liquor. This effect: is much accelerated by frequently opening the phial, but re- 

 tarded by the addition of alcohol. 



3. Elemi fucceeds as well, or better. The fame obfervations are applicable when this is 

 ufed, as in the preceding cafe. The precipitate, however, when formed, fubfides to the 

 bottom of the liquor. It is more bulky, and not fo hard as that of th< maftic. It is not 

 unlike that which is formed in fuch ink as contains much gum and alum. 



4. A mixture of elemi and maftic in folution feems much fuperior to either ; and would 

 perhaps remain fufpended any length of time, if the ingredients were fo apportioned as to 

 make the compound precipitate as nearly as poffible of the fame fpecific gravity as the liquor. 

 The beft produft was obtained nearly as follows ; but the ingredients not having been ac- 

 curately v.-eighed, the quantities are only given by eftimation. — Digeit ten or twelve grains 

 of the -whiteft pieces of mallic, felecled for this purpofe and powdered, in two ounces of 

 alcohol -, and, when nearly diffolved, add twenty grains of elemi. When both the refuis 

 are dilTolved, add ten or fifteen drops of reftified oil of amber, and fifteen or twenty of 

 efl'euce of bergamot : fliak« the whole well together, and let the faeces fubfide. The folu- 

 tion will be of a pale amber colour. It is to be added in very fmall portions to the bed 

 aqua ammonia: purse, until it afiumes a milky whitenefs — fhaking the phial well after each 

 addition, as diredled by Macquer. The ftrength and caufticity of the ammoniac are of 

 mod eflential confequence. If, upon the addition of the firft drop or two of the tiniflure, 

 a denfe opake coagulated precipitate is formed — not much unlike that which appears on 

 dropping a folution of filver into water (lightly impregnated with common fait — it is too 

 ftrong, and muft be diluted with alcohol. A confiderable proportion of the tincture, per- 

 haps one to four, ought to be requifite to give the liquor the proper degree of opacity. 

 I have frequently fince made an eau de luce in this way, in which no depofition has after- 

 wards appeared 



J. The refin formed from oil of amber by the nitric acid was, after boiling it in water, 

 tried for this purpofe. It was itfelf of a light chocolate colour, and produced an opake 

 fawn-coloured liquor. Common yellow refin formed a coagulum like that of maftic, but 

 almoft inftantly. Its colour was bad, and its factor too ftrong to be difguifed, 



6. Balfam of copaiba was tried, and appeared to be a very good fubftance for this pur- 

 pofe ; but fome other objects of enquiry intervening, the experiments were difcontinued. 

 Copal is, it is faid, equal, if not fuperior, to any of the refins ; particularly on account of 

 its having a lefs unpleafant odour than moft of them. I have not tried it myfelf ; but, 

 from the defcrved reputation of fome trading chemifts who I know make their eau de luce 

 of it, I have no doubt of its unfwering extremely well *. 



I am, Sir, your moft obedient Servant, 

 I2th June 1797. J. F— :— :— :_:_n. 



* The theory of opake HuiJs depends on confiderations of an optical, mech.->nical, and chemicnl nature. For, 

 ,. Such a fluid muft confill of pam that differ in their rcfraftivc power, and arc not fmallcr than a determinate 

 magnitude. Ncwion't Optics, HI. Prop. 4. ». Tlic dcnfcr particles will dcfccnd with velocities nearly uni- 



form, 



