166 Scientific Intelligence. 



Hence it indicates the moment when an alkaline solution is saturated^ 

 by an acid, for it remains blue as long as there is any free alkali in 

 tlie solution, and acquires the color of onion peelings as soon as the 

 acid is slightly in excess. 



" Litmus may serve equally well for detecting the presence of an 

 alkah. If the blue paper be passed through water containing a few 

 drops of an acid, it is reddened, and then will be restored to a blue, 

 by a very small quantity of alkali." To determine the neutral point 

 of a saline compound, a small glass rod may be dipped into the solu- 

 tion, and a mark drawn over the litmus paper. To distinguish be- 

 tween the change of color occasioned by the liberation of carbonic 

 acid, and that produced by a stronger acid, it is observed that the for- 

 mer gives a wine-red, while the latter produces that of onion peelings, 

 ^vhich is permanent. — Arm. de Chimie et de Phys. Decern. 1828. 



2. Liquid Sulphurous acid. — In an interesting memoir on the con- 

 densation of sulphurous acid gas, by Auguste de la Rive, of Ge- 

 neva, the author, after some judicious remarks on the distinction be- 

 tween gases and vapors, observes, that in order to obtain liquid sul- 

 phurous acid, the gas must be as dry as possible, before it is condens- 

 ed. For this purpose it should be received as it arises from the ac- 

 tion af sulphuric acid on mercury, in a vessel No. 1, surrounded by 

 a freezing mixture ; and from this it should pass through a lube filled 

 with well dried muriate of lime, into a vessel No. 2, surrounded like 

 the first : and lastly, the portion which has not then been condensed, 

 should pass through a second tube, filled with muriate of lime, into 

 a third vessel, cooled like the others, when the condensation is finish- 

 ed. A tube may proceed from this third vessel into mercury, which 

 occasions a slight pressure of the gas that might have escaped con- 

 densation, from spreading in the chamber, and compels it, like the 

 rest, to become liquid. Care must be taken to lute the junctures 

 well. After the gas has been disengaged, during eight or ten hours, 

 there are found in No. 1, white crystals, composed of water and sul- 

 phurous acid, and in Nos. 2 and 3, liquid sulphurous acid, very pure, 

 which must be immediately corked up hermetically in a flask. To 

 preserve it, the flask must be constantly surrounded with a freezing 

 mixture, otherwise the gas will either escape through the cork, or it 

 will explode the vessel. 



The crystals of sulphurous acid and water, formed in the first ves- 

 sel, are of a beautiful white, and have an agreeable, acid taste. They 



