22 SUBSTANCES SOLUBLE LN PETROLEUM SPIRIT. 



drawn through the apparatus, entering by the tube b, and 

 passing out through a. A chloride of calcium tube (c) is placed 

 before b and another between a and B, the former to dry the air 

 entering the apparatus, the letter to prevent moist air from the 

 aspirator passing into A. These precautions are necessary, for 

 if the atmosphere in which the evaporation of the petroleum 

 spirit is to take place be not completely dried, moisture may be 

 deposited on the glass dish containing the solution, in consequence 

 of the cold produced by evaporation, thus causing, of course, an 

 increase of weight. It is advisable, the^refore, to connect the first 

 chloride of calcium tube (c) with a WolfiPs bottle one-third full of 

 concentrated sulphuric acid. A current of air is then passed 

 through the apparatus and the petax)leum spirit allowed to 

 evaporate at the ordinary temperature until the operation 

 appears complete; that is, until the residue has only a sli^t 

 smell of petroleum spirit. The glass dish is then closed and 

 weighed. After the weight has been accurately taken, it is again 

 opened, exposed for one minute to the air, closed and again 

 weighed, and the alternate exposure and weighing repeated until 

 the same loss in weight is observed twice in succession. This 

 loss is then assumed to be the amount of ethereal oil that diSuses 

 into the air per minute. It is also assumed that during every 

 previous exposure of one minute the same weight of ethereal oil 

 has evaporated, so that to the quantity of oil as found by the 

 last weighing there has to be added the ' co-eiRcient of evapora- 

 tion,' multiplied by the number of minutes the dish has been 

 exposed. (Comj^are the examples of estimation in § 136.) If the 

 coef35cient of evaporation is less than one milligram this cor- 

 rection may be omitted. Perhaps it would be advantageoxis to 

 pass a current of carbonic acid through the apparatus during the 

 evaporation of the petroleum spirit, as many ethereal oils diffuse 

 much more slowly into that gas than into atmospheric air. 



§ 23. In Presence of Fat and Besin. —After thus determining 

 the weight of the substances dissolved in a known quantity of 

 petroleum spirit, it must be ascertained whether the residue after 

 evaporation ia completely volatile at 110°, or leaves a non-volatile 

 residue of resinous or fatty matter. In the latter case the weight 

 must be determined and deducted from that obtained in § 22. 

 (See § 138.) If the non^volatile part constitutes the majority of 

 the dissolved substances it may be ascerteined, after the removal 



