310 LIQUIDS AND GASES. 



in (J alcohol at the ordinary temperature with a pull equivalent to a 

 negative pressure of 25 atmospheres, by completely lillini;- a bulb with 

 alcohol, and then cooling it. The alcohol in contracting- strains the 

 bulb inwards; and iinally, when the tension becomes very great, parts 

 from the glass with a sharp "click." 



To realize a portion of the other bend of the curve, an experiment has 

 been devised by Mr. John Aitken. It is as follows: If air — that is 

 space, for the air plays a secondary part — saturated with moisture be 

 cooled, the moisture will not deposit unless there are dust particles on 

 which condensation can take place. It is not at first evident how this 

 corresponds to the compressing of a gas Avithout condensation. But a 

 glance at the figure will render the matter plain. Consider the isother- 

 mal 175° 0. for ether at the ])oint marked A. If it were possible to lower 

 the temperature to 100° C. without condensation, keeping volume con- 

 stant, the pressure would fall, and the gas would then be in the state 

 represented on the isothermal line 160° at G, — that is, it Avould be in 

 the same condition as if it had been compressed without condensation. 



You saw that a gas, or a liquid, is heated by compression ; a piece of 

 tinder was set on fire by the heat evolved on compressing air. You 

 saw that condensation of ether was brought about by diminution of 

 pressure — that is, it was cooled. Now, if h,ir be suddenly expanded 

 it will do work against atmospheric pressure aiul will cool itself. This 

 globe contains air; but the air has been fitltered carefully through cot- 

 t(m-wool, with the object of excluding dust particles. It is saturated 

 with moisture. On taking a stroke of the pump, so as to exhaust the 

 air in the globe, no change is evident; n.o condensation has occurred, 

 although the air has been so cooled that the moisture should condense 

 were it possible. On repeating the operation with the same globe, 

 after admitting dusty air — ordinary air from this room — a slight fog 

 is produced, and, owing to the light behind, a circular rainbow is 

 seen; a slight shower of rain has taken place. There are compara- 

 tively few dust particles, because only a little dusty air has been ad- 

 mitted. On again repeating the fog is denser; there are more par. 

 tides on which moistnre may condense. 



One point more and I have done. Work is measured by the distance 

 or height through which a weight can be raised against the force of 

 gravity. The British unit of work is a foot-pound — that is, a pound 

 raised through 1 foot; that of the metric system is 1 gram raised 

 through 1 centimeter. If a pound be raised through 2 feet twice as 

 much work is done as that of raising a pound through 1 foot, and an 

 amount e(pial to that of raising 2 pounds througli 1 foot. The measure 

 of Avork is therefore the weight nuiltij)lied by the distance through 

 which it is raised. When a gas expands against pressure it does work. 

 The gas may be supposed to be confined in a vertical tube and to pro- 

 l)el a i)iston upward against the pressure of the atmospheie. If such 

 a tube has a sectional area of 1 square centimeter, the gas in expand- 



