130 SEC. 5. MOLECULAR PHYSICS. 



584. Hydrometer (No. 10), for preparing a liquid having at 

 15 C. the specific gravity of pure water at 4 C. The instrument 

 is made in the following manner : Through the open stem shot 

 is introduced till the instrument floats at the mark on the stem in 

 pure water of 15 C. The weight of the instrument is then 

 increased in the ratio 0-99915 : 1, in consequence of which it floats 

 at the same mark at 15 C. in a liquid having at that temperature 

 the specific gravity of pure water at 4 C. 



The thermometers have the Celsius' scale. By their mode of 

 construction they possess the following advantages : (1.) They 

 may be turned upside down and shaken in any manner without 

 breaking the column of mercury. (2.) Though newly made, the 

 zero is not subject to displacement. 



The former advantage is obtained by filling the tube and the 

 upper recipient with perfectly dry, dust free air, as highly com- 

 pressed as possible. 



The latter advantage is secured by placing the bulbs of the 

 newly made thermometers in a bath of paraffin, heated slowly to 

 100 C. and then allowed to cool slowly and in succession sixty to 

 a hundred times. 



585. Trough for comparing Hydrometers. 



II. AIR PUMPS AND PNEUMATIC APPARATUS. 



586. Hand Pump, Regnault's System improved. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



Sucking and forcing hand pump. The hand purnp, constantly used in 

 laboratories, has now attained a satisfactory practical shape, from which not 

 much deviation is possible. Its general proportions are determined by the 

 consideration of what best use can be made of muscular power, and also of 

 the simplicity and facility of carriage of the apparatus. Silken valves, as 

 being too perishable, are excluded and replaced by cones of metal and leather. 

 All the moveable parts, piston and valves, are equally accessible. Three 

 cocks effect the complete cutting off of all communication between the pump 

 and the exhaust pan and the compression pan, as well as restore direct 

 communication, either between both or one of these and the atmosphere. If 

 rarefaction is required in the compressing vessel, or vice versd> without dis- 

 turbing the tubes of communication, the relative position of the valves must 

 be inverted, which may be done in a few minutes, or else a commutative 

 cock must be joined to the pump. 



For effecting all communication between the pump and its recipients, a 

 screw flush joint is always used, which is very safe, and can be adjusted to 

 tubes made of india-rubber, copper, or lead. This avoids the deterioration of 

 the india-rubber, which is unavoidable on account of the constant action of 

 the fastening joints, especially as it is rarely that a thoroughly air-tight joint 

 can be made with india-rubber alone. 



