436 Dr, Thomas Thomson, on [Dec. 



tion which has taken place in its price, together with the 

 great improvement in its quality, has contributed very much, 

 indeed, to its almost universal employment. It is suffici- 

 ently known, that the mode of making this important article 

 of commerce is to expose slacked lime, in the state of a dry 

 powder, to an atmosphere of chlorine gas, till it ceases to 

 absorb it. Unslacked lime does not possess the property of 

 absorbing this gas. 



When Mr. Dalton published his experiments on bleaching 

 powder, in 1813, (Annals of Philosophy, ii. 6.) he found it 

 a compound of 1 atom chlorine and 2 atoms lime. And 

 this seems to have been its usual strength about that period. 

 When such a powder is digested in water, only half the 

 the lime dissolves in combination with the chlorine ; the 

 other half, (abstracting the small quantity of quick lime 

 soluble in water) remains undissolved. But, of late years, 

 the strength of the powder has been so much increased that 

 it is nearly all soluble in water. Such powder contains one 

 atom of chlorine to every atom of lime in the compound. 

 The best bleaching powder manufactured in Glasgow is of 

 this quality ; and I have analyzed it from Belfast equally 

 strong. 



It is well known that bleaching powder is white and 

 pulverulent. Its taste is hot, bitter, and astringent, and it 

 has a peculiar smell. When digested in water it always 

 leaves behind a little carbonate of lime, mixed with some 

 silica and a very little sand. These things I consider as im- 

 purity from the lime employed, which is never absolutely 

 free from foreign matter. 



When bleaching powder, dissolved in water, or mixed 

 with water in such proportion as to be in the state of a paste, 

 comes in contact with sal-ammoniac, nitrate of ammonia, 

 oxalate of ammonia, or probably any ammoniacal salt, a 

 violent effervescence takes place, and azotic gas is evolved 

 in abundance. This evolution is occasioned by the decom- 

 position of the ammonia, the hydrogen of which unites with 

 the chlorine, (and, doubtless, also with the oxygen), con- 

 verting it into muriatic acid and water, while the azotic gas 

 is disengaged. For every atom of ammonia decomposed, 

 three atoms of chlorine are converted into muriatic acid. 

 When the solutions are too dilute, very little azotic gas is 



