248 ESSAY XXIII. 



precipitation (for express mention is made of an efferves- 

 cence), has the properties of an acid, and can therefore 

 dissolve part of the lime. The lime dissolved in this 

 manner will not fall down until a good quantity of alkaline 

 spirit is added ; and why is so much required ? because it 

 is the caustic part only of this spirit which effects a 

 precipitation, as it has a stronger attraction for the super- 

 fluous fixed air which holds the earth in solution. His 

 supposed reciprocal attraction has no place here. Alkaline 

 spirit obtained from horns and hoofs should never be used 

 in exact chemical experiments. That from common sal 

 ammoniac is the best for these purposes ; and this is the 

 reason why the author, after he had saturated the spirit 

 from hoofs with muriatic acid, and distilled the ammoniacal 

 salt with spirit of vitriol, obtained at the last a volatile spirit 

 in the receiver which effervesced with acids. Had the 

 author distilled pure but common sal ammoniac with oil of 

 vitriol, his volatile spirit would certainly not have effervesced 

 with acids. But why did the spirit obtained by the author 

 effervesce ? because it contained some oily matter, which was 

 destroyed by the vitriolic acid on the application of the heat ; 

 as, during the decomposition of oil, fixed air is always 

 separated (let any one distil oil of vitriol with a little fat, 

 he will easily discover fixed air in the receiver) ; and as it 

 combines with the spirit driven over into the receiver, the 

 spirit must of course effervesce with acids. That the 

 vitriolic acid should remain in the retort is owing to its 

 fixity, a property which it has in common with other known 

 acids, which are more or less fixed as the phosphoric and 

 arsenical acids, and sedative salt. The author certainly 

 does not know that a quantity of fixed air is separated 

 from coal during its combustion, as being one of its 

 constituent parts, otherwise he never would have related 



