440 An Easy Met hod of [Dec. 



less than in recently made powder. For example, in a speci- 

 men analyzed, the atoms of chlorine in the powder were to 

 the atoms of oxygen, disengaged very nearly as 6 to 5. And 

 in other and older specimens, the chlorine bore a still higher 

 ratio to the oxygen. This must be owing, I conceive, to a 

 tendency which chlorous acid must have to part with its 

 oxygen, and the chlorine thus left at liberty gradually con- 

 verts the lime, with which it is combined, into calcium. 

 And, were the powder kept long enough, it would doubt- 

 less be completely changed into chloride of calcium by 

 length of time. When such a change has taken place the 

 bleaching properties of the powder are, of course, at an 

 end. Many years ago, when I lived in London, I met with 

 specimens of bleaching powder on sale in some of the che- 

 mists shops very nearly effete. In these specimens almost 

 the whole combined lime had been converted into calcium, 

 and, of course, the salt into chloride of calcium. 



In the preceding analysis I have not noticed a small quan- 

 tity of manganesic acid which bleaching powder always con- 

 tains. If you raise a solution of bleaching powder to the 

 boiling temperature, it assumes the beautiful red, or rather, 

 purple colour which characterizes solutions of manganesic 

 acid, and some of its salts. If the liquid be exposed to the 

 light of the sun it gradually loses its colour, while, at the 

 same time, a slight deposite of a black powder, obviously 

 oxide of manganese, takes place. The quantity of this acid 

 present in bleaching powder is small ; but, so far as my 

 observations extend, it is never absent. 



Article IV. 



An Easy Method of Filling Barometers. By a 

 Correspondent. 



An accurate barometer is essential in gaseous investigations ; 

 but as boiling the mercury in the tube is rather hazardous, 

 and the fitting it on to an air-pump, a work of time and 

 attention; such an instrument is troublesome to make, or 

 expensive to purchase. Advantage may, however, be taken 

 of the vacuum produced in the barometer itself, and a cor- 



