WATER. 213 



The refractive power of water is very high, owing, as is supposed, 

 to the hydrogen which it contains. By a vigorous stroke in a syringe, 

 water emits a flash of light. Thenard. 



Water has generally been regarded as incompressible, but Mr. Per- 

 kins applied to it a force of 2000 atmospheres, and stated the com- 

 pression at y 1 ,-, but Prof. Oersted* justly considers this estimate as far 

 too great. It would appear from a note by the late Prof. Fisher,f 

 of Yale College, that the subject is not quite new, and Mr. Canton, 

 so far back as 1764, ascertained that water expands ^TTT o P art > by 

 the removal of the pressure of the atmosphere, and that an additional 

 atmosphere reduces its volume in an equal degree. No natural water 

 is quite pure ; it always holds saline and earthy matters dissolved be- 

 sides gases; rain or snow water obtained away from population, as on 

 a mountain, is the purest. It is obtained pure by distillation, espe- 

 cially in vessels of gold, silver, or platinum. Distilled water is indis- 

 pensable in all accurate chemical operations. 



8. Utility of water. It is far more abundant than all other fluids ; 

 it is indispensable to animal and vegetable life, and no other fluid 

 would answer the same purposes. 



Water enters into the composition of all the solids and fluids which 

 we consume for food and drink ; it imparts that humidity to the air 

 which in breathing moderates animal heat ; it affords by its pressure 

 and motion, the means of great mechanical operations, and it facilitates 

 commerce and friendly communication between nations. It is ne- 

 cessary that its properties should be negative, or it would be injurious. 



Gazometer for oxygen or any gas not absorbed by water. Dr. Hare. 



" The engraving on p. 214, represents a section of the gazometer 

 for oxygen, which is capable of holding between five and six cubic 

 feet of gas. It is placed in the cellar beneath the lecture room. The 

 wooden tub, V, is necessarily kept nearly full of water. The cylin- 

 drical vessel, T, of tinned iron, is inverted in the tub, and suspended 

 and counterpoised, by the rope and weight, in such manner, as to re- 

 ceive any gas which may proceed from the orifice of the pipe, in its 

 axis. This pipe passing, by means of a water-tight juncture, through 

 the bottom of the tub, rises through the floor, F, is furnished with a 

 cock at C, and terminates in a gallows screw. This is fixed in a 

 cavity made in the plank forming the table of the lecture room, in the 

 vicinity of the pneumatic cistern. Hence by means of it, and a lead- 

 en pipe soldered to a brass knob, properly perforated, a communica- 

 tion may be established between the cavity of the gazometer, and any 

 other vessel, for the purpose either of introducing or withdrawing the 

 gas. The counter-weight being made heavier than the vessel, by 

 appending additional weight to the ring, K, the gas may be sucked 



* Edin. Jour. No. 12, p. 201. t Am. Jour. Vol. Ill, p. 347. 



