258 ESSAY XXIV. 



crystallisation of the neutral salt prepared from nitre and 

 arsenic ; and as Mr. W. could not have been acquainted 

 with arsenic and its constituent parts, he is not to be 

 blamed for entertaining such an opinion. 



P. 379. The fixing of arsenic by caustic spirit of sal 

 ammoniac is truly remarkable. But it is certain that 

 caustic volatile alkali always contains some lime. It 

 appears, when it is kept long in glasses that are frequently 

 opened, in the form of a hard pellicle deposited on the 

 glass. Is it not this lime which adheres to the arsenic 

 and fixes it ? If not, and the experiment be exact, the 

 problem is one of the most difficult in chemistry. 



P. 404. It is said that volatile alkali easily dissolves 

 precipitated magnesia ; but the solution is entirely owing 

 to the water in which alkali is dissolved. The separation 

 of the earth, when the solution is exposed to the open 

 air, arises not from the evaporation of the volatile alkali, 

 but from the escape of the aerial acid which held the 

 earth in solution in the water; for from 1 Ib. of Epsom 

 salt, dissolved in 64 Ib. of water, no precipitate can be 

 obtained by pure potashes without boiling ; but when the 

 air is driven off by boiling, the magnesia is obtained. 



P. 423. " Fixed alkalies precipitate a solution of silver 

 prepared with volatile alkali." How is that possible ? It 

 is indeed true that a precipitation takes place ; but what 

 a great quantity of fixed alkali must be added before all 

 the silver is precipitated. With what does the alkali 

 combine ? It has no attraction for the volatile alkali ; 

 and, without a new combination, it is impossible to conceive 

 a precipitation. Saturate pure salt of tartar with abstracted 

 vinegar, or even precipitated nitrous acid, and you will 

 find that the solution of silver yields some precipitate. I 

 have never met with alkali which did not show some 



