On Sal Nitruin and Nitro-A'erial Spirit 39 



virtue of which terrestrial nitre is produced and the 

 sulphur brought to a suitable volatility. 



It follows from what has been said that the salts of 

 which plants are composed are to some extent nitrous 

 and not purely saline, as we intimated above. For all 

 vegetable salts are derived either from the air or from 

 the earth. As regards the air it is by no means to be 

 supposed that an alkaline and fixed salt resides in it ; 

 nor is the earth impregnated with a purely saline salt, 

 for only nitrous salts can be extracted from it by lixi- 

 viation. And hence we may conclude that the salts 

 of plants are nitrous and not purely saline. Hence 

 in soil on which plants grow abundantly no nitrous 

 salt is to be found, the reason being that all the nitre 

 of the soil is sucked out by the plants. But when 

 plants are calcined to ashes, the acid spirit of the 

 nitre of which they are composed goes off as vapours, 

 while the other element of the nitre — to wit, the 

 alkaline salt — is left in the ashes. And hence it is 

 that plants yield a greater quantity of fixed salt 

 when burned fresh and with the least possible flame ; 

 but this does not result as some suppose because the 

 alkaline salt goes off as vapours when the plants 

 are slightly dried, for it has an exceedingly fixed 

 nature and remains undiminished and intact in the 

 hottest fire. But when plants containing much sul- 

 phur are dried and then burned in a bright flame, 

 the sulphureous parts, burning with a fiercer flame^ 

 kindle the nitrous salt, and carry the whole of it 

 away with them as vapours, very much as when 

 gunpowder is ignited. If, however, green herbs are 

 calcined with the flame kept down, their volatile 

 sulphur, together with the original moisture, passes 

 into smoke and goes away, while the nitrous salt 

 remains behind ; but if calcined in a hotter fire, the 



