392 Intelligence and Miscellaneous Articles. 



cident light, the intensity of reflexion being a minimum at an inter- 

 mediate incidence ; and if we suppose these quantities to be really- 

 equal at the incidences observed by Bouguer, we must take the mean 

 of all his numbers, which is *69, as the most probable result of ob- 

 servation. This result differs but little from one of the two numbers 

 given by him at each incidence, and scarcely at all from the result 

 of calculation. 



The angle at which the intensity of reflexion is a minimum, when 

 common light is incident, may be found from the formula 



( m +m) (c + j) = ( M -w) t ifl + »'>- 4cos * « 



which gives the value of p, and thence that of i. This incidence for 

 mercury is, by calculation, 75° 15', and the minimum value of I is 

 •693, which is less than its value at a perpendicular incidence by 

 about one-eighteenth of the latter. According to the formula?, the 

 reflexion is always total at an incidence of 90°. 



LVIII. Intelligence and Miscellaneous Articles. 



ON ABSINTHIC ACID. BY C. ZWENGER. 



ACCORDING to M. Braconnot, wormwood (Artemisia absin- 

 thium, Linn.) contains a peculiar deliquescent uncrystallizable 

 acid, the ammoniacal salt of which crystallizes in four-sided prisms ; 

 he has named it absinthic acid. 



To obtain this acid in a pure state, a decoction of wormwood 

 (stalks, leaves and flowers) is to be treated with excess of a solution 

 of acetate of lead ; a bulky precipitate of a dirty yellow colour is 

 formed ; the supernatant liquor contains some of the salt in solution, 

 which is very soluble in free acid ; and ammonia is to be added to it 

 until it is only slightly acid ; the salt of lead, after being well washed, 

 is to have three or four times its bulk of water added to it, and then 

 to be decomposed by hydrosulphuric acid ; and it is requisite that 

 the vessel which contains the salt of lead should be kept at 140° 

 to 158° Fahr., in order that the decomposition may be complete. 

 The liquor separated by the filter from the sulphuret of lead is to be 

 again precipitated by acetate of lead, and the precipitate obtained is 

 to be decomposed in the same manner. The filtered liquor is after- 

 wards evaporated to the consistence of a syrup, and the residue is to 

 be treated with hot aether until there is no acid reaction ; the aether 

 is to be separated by distillation, and water is to be poured on the 

 brown residual mass ; by this an acid resin is precipitated, which, 

 after a little time, forms a compact deposit on the sides of the vessel ; 

 this resin is the bitter principle of the wormwood. 



The aqueous solution is of a yellow colour, and yields crystals by 

 evaporation, which are purified with difficulty, either by pressure 

 between sheets of filtering paper or by repeated crystallization. As 

 this acid is volatile, it is best purified by dry distillation ; the greater 

 part of the empyreumatic oil which rises with it may be separated 

 by the addition of water ; the crystals thus obtained are more readily 



