116 ESSAY VI. 



as soon as the mass enters into fusion ; this acid will be very 

 volatile. If the distillation be continued with a strong fire 

 till no more acid goes over, the residuum in the retort 

 proves to be very alkaline. If a solution of hepar sulphuris 

 be mixed with this imperfect neutral salt, the sulphur will 

 be precipitated ; in like manner, a solution' of soap is rendered 

 turbid by it. It is hence evident that the acid of arsenic 

 may be combined with a greater quantity of alkali than is 

 necessary to its saturation ; and that, in consequence of this, 

 the nitrous acid is expelled, whence an arsenical salt, with 

 a superabundance of alkali, arises. This attraction, however, 

 has likewise its limits ; for when a mixture of one part of 

 acid of arsenic and six parts of nitre is melted in a covered 

 crucible in a strong fire till all ebullition ceases, there will 

 be found, besides the arsenical salt, a considerable quantity 

 of undecompounded nitre. But concerning the alkalisation 

 of nitre with arsenic, there is one principal point to be 

 noticed, viz. that nitre always attracts a certain small 

 quantity of phlogiston from the heat itself, and that this 

 phlogisticated nitrous acid forms with the alkali but a loose 

 union, so that vegetable acids are able to expel it from 

 its basis. This property of nitre is the reason why acid of 

 arsenic, by virtue of its attraction for the superfluous alkali, 

 expels the nitrous acid phlogisticated by heat ; whence it 

 likewise follows that, since common salt undergoes no 

 change in the fire, the acid of arsenic is not able to alkalise 

 it, as will appear from the following paragraph. 



SECTION XL MURIATED FOSSIL ALKALI, OR COMMON SALT. 



One part of common salt was distilled with three parts 

 of acid of arsenic. Water first went over ; when the mass 

 was dry, I changed the receiver, and increased the fire till 



