266 Researches respecting the radical of Benzoic Acid. 



with* altogether, since the tightness may be judged of after a little prac- 

 tice, by the force with which the air rushes in through the opened cock 

 d y after exhaustion. 



To the table is screwed a strong wooden post e, Fig. % on which 

 the pump is fastened by its screw. The moisture contained in the 

 oxide of copper mixture is expelled at the same time with the air by 

 exhausting the ignition tube, from which by degrees the last trace is 

 removed, since the air dried by the chloride of calcium is often ad- 

 mitted by repeated exhaustions and opening of the stop-cock. 



It is evident that the expulsion of moisture may be hastened, from 

 substances from which we have to fear no loss by warmth, if the 

 ignition tube be put into a tin tube filled with hot water.* 



This small air pump presents yet another advantage, of which we 

 frequently availed ourselves in our experiments. The oil and other 

 fluids submitted by us to analysis possess so high a boiling point, that 

 the small bulb filled with them, was emptied of the portions of the 

 liquid, not before this part of the tube had almost attained a red heat. 

 It thence frequently happened, that the gas was suddenly evolved with 

 such violence, as to throw some oxide of copper into the chloride of 

 lime, and thereby the experiment became unavailing, at least for the 

 determination of hydrogen. This is completely avoided by turning 

 the open end of the small bulbs towards the closed end of the ignition 

 tube, introducing the oxide of copper in layers and then exhausting. 

 The small bubble of atmospheric air in the bulbs, now suffices to ex- 

 pel all the contained moisture, particularly if the ignition tube be 

 brought to a more vertical position, and exhaustion be repeated. In 

 order not to mention the accuracy of the result disadvantageous^, 

 it may be added that this manipulation with very fluid substances is 



Ignited with these 



throughout superflous. 



We return to the pure oil of bi 

 precautionary measures, it yielded, 



I. 0.366 gramme*= 1.109 carbonic acid, and 0.200 water. 

 II. 0.341 " =0.982 " " 0.175 " 



* This pump may be used with great convenience for drying substances which 

 suffer drying only in a vacuum, at the common or a slightly raised temperature. 

 In place of the ignition tube, a short tube is fastened on which is closed beneath, or 

 a small glass globe, in which is placed the substance to be dried. 



t The gramme is always to be understood as the weight or series of weights em- 

 ployed. 



