42 



SCIENTIFIC AMUSEMENTS. 



necessary to boil down to a small bulk at as low a temperature as 

 possible to avoid damage by heat. In order to do this a powerful 



pump is connected with 

 the boiler so as to pump 

 out the vapour as fast as 

 it is formed, and always 

 maintain in the boiler only 

 a very small degree of 

 pressure. This arrange- 

 ment is usually termed a 

 Vacuum Pan. 



By rapidly removing 

 the vapour of water as fast 

 as it is evolved, it is pos- 

 sible to chill the water by 

 its own evaporation so 

 much that ice forms on 

 the surface. 



Expt. 36. To make 

 Distilled Water. For 

 small quantities, obtain 

 a stoppered glass retort 

 holding about a pint (fig. 



Fig. 21. "Water made to boil by pouring 

 Cold Water on Flask. 



22), half fill it with water, 

 place it on a stand, and 

 heat it to boiling over a gas or spirit-lamp. The steam produced 

 passes over into the neck of the retort, and there condenses to 

 drops which run down into a long-necked flask which acts as a 

 receiver. In order to obtain a more extended cooling surface the 

 receiver may conveniently be placed in a basin of water and its 

 upper surface kept moist by means of a piece of cloth, &c., laid over 

 it and dipping into the water in the basin; or better still, as indi- 

 cated in fig. 22, a small stream of water is allowed to trickle over 

 the top of the receiver by means of a flexible tube connected with 

 a water tap. A yet more effective method is to interpose a water- 

 jacketted condenser (often called a Liebig's Condenser) between the 

 retort and the vessel into which the condensed water drops ; fig. 23 

 indicates such an arrangement of retort and condenser, the water in 

 the jacket being continually renewed by cold water which runs in 

 at the lower end of the condenser from a convenient vessel (or from 

 the water-main by means of a flexible tube), whilst the heated water 

 issues from the upper end and flows into another receptacle (or 

 directly into the sink). A large glass flask or tin can may be con- 

 veniently substituted for a glass retort, the mouth being fitted with 



