10 PHILOSOPHICAL TRANSACTIONS. [anNO 1735. 



pole B. And hence also it appears, how a line of the 3d order may be described 

 through 7 given points, one of which may be double. 



Let the angles oan, obn, be moved as before, about the given points a, b, 

 (fig. 10) ; and through the intersection o, of the legs oa, ob, let the line ocp 

 pass, drawn from another given point c, meeting the side an, of the angle a, 

 in p ; then through p, and a 4th given point d, draw the line dpq, meeting 

 the leg Ao in Q ; then the point a will describe a line of the 4th order, having 

 a triple point in the pole a. 



And thus, by increasing the number of the poles a, b, c, d, &c. so that their 

 number at length may be n, the line described will be of the same order n. But 

 it may be noted, that if for the angle obn, there be substituted a right line, re- 

 volved about the pole b, the description will become easier. 



j4n Account of M. Seignettes* Sal Polychrestus Rupellensis, and some other 

 Chemical Salts. Bij M. Geoffrey. N" 436, p. 37. 



M. Seignette's sal polychrestus rupellensis is a soluble tartar, composed of 

 cream or crystals of tartar, and the fixed salt of the kali of Alicant,-|- well de- 

 purated. This salt is very singular: for, though it be a fixed alcaline salt, it 

 has the peculiar property of crystallizing; nor does it easily dissolve in the open 

 air ; as other fixed salts do ; but on the contrary calcines in it like vitriols or 

 Glauber's salt. Another peculiar property M. GeofFroy observed of it is, that if 

 it be saturated with vitriolic acid, and the liquor be evaporated, there results a 

 salt that resembles Glauber's salt, and has all the properties requisite to make 

 M. Seignette's salt. 



To produce this, take of the salt of kali [of Alicant] well depurated, 1 lb ; 

 dissolve it in water, add about 1 lb. ss. of crystals of tartar ; boil the whole, to 

 dissolve the crystals : but the exact proportion of crystals of tartar can be de- 

 termined no more in this operation, than in making the soluble tartar ; either 

 because the salt of kali has retained more or less humidity in its crystallization, 

 or because the tartar has more or less impurities in it. But if there be too much 

 tartar in the alkaline liquor, after the fermentation is over, filtrate the liquor, 

 and as it cools, the superfluous tartar will fall to the bottom. After the sepa- 

 ration of the tartar from the liquor, evaporate the lixivium by a gentle fire ; set 

 it in a cool place to crystallize, and you will have very fine crystals. If the 



* M. Seignette was an apothecary at Rochelle, whence this salt has been called Rochelle salt. 



t The kali of Alicant is a marine plant (Salsola sativa Linn.) from which, by incineration, is ob- 

 tained the fossil alkali or soda, which uniting with the superfluous acid of the crystals of tartar, forms 

 a triple salt, compounded of tartaric acid, potass and soda, and denominated in the modern chemical 

 nomenclature tartrite of potass and soda. 



