LABORATORIES FOR GENERAL TEACHING 27 



blocks set in concrete, and immediately beneath and easily 

 accessible are channels in which gas and water pipes, etc., are 

 laid, as well as open troughs by which the waste liquids from 

 the sinks above are delivered into the drain. Obstructions 

 resulting from the accumulation of solid matters in pipes are 

 thus avoided. 



From the main laboratory a few feet away across the corridor 

 is the laboratory or rather series of laboratories devoted to 

 physical chemistry. Here the general arrangements of working 

 tables, fume chambers, light and heat, water and drainage are, 

 in all essential particulars, the same as in the large laboratory. 

 But some of the special arrangements deserve notice. One of 

 the first of these is the room for gas analysis. It would be im- 

 possible in this place to describe the several forms of gas ap- 

 paratus employed for purposes of this kind, and the chemical 

 student must be referred to one of the special technical treatises 

 which deal with the subject, which for successful handling 

 requires experience, and accordingly it is usually assigned to a 

 comparatively late stage of the course of instruction. 



It is sufficient to say that as mercury is much used, and is 

 expensive, it is necessary to provide for collecting it in case of 

 its being spilt : the floor therefore is smoothly cemented and 

 slopes very slightly to channels which lead into a small reservoir. 

 In the next room is a table specially fitted against the wall with 

 connections to a battery of storage cells, and resistances by 

 which a current of any desired strength or voltage can be used 

 for the purpose of electrolytic deposition of metals or for 

 other purposes to which the electric current is now con- 

 veniently applied, especially to oxidation or reduction in liquid 

 media. 



Distillation under greatly reduced pressure is a method of 

 purification often applied to substances which cannot be heated 

 to their boiling-points under ordinary atmospheric pressure 

 without suffering decomposition. Accordingly the means of 

 obtaining a pretty good vacuum is frequently needed, and in 

 the physical laboratory is installed a rotary pump, electrically 

 driven, and capable of reducing the pressure in a vessel of 

 moderate size to about 5 millimetres of mercury or less. The 

 pump is connected with a large vessel placed in the area outside 

 so that slight leakage of air through taps or joints may not 

 appreciably disturb the vacuum, which is laid on by means of a 



