372 Professor W. Ramsay [May 8, 



November IStli, "The experiment for the high suspension of quick- 

 silver being called for, it was found that it had failed. It was ordered 

 that thicker glasses should be provided for the next meeting." 



There can be no doubt that this behaviour is caused by the attrac- 

 tion of the molecules of the liquid for each other. And if the 

 temperature be sufficiently low, the pressure may be so reduce^ that 

 it becomes negative — that is, until the liquid is exposed to a strain or 

 pull, as is the mercury. This has been experimentally realised by 

 M. Berthelot, and by Mr. Worthington, the latter of whom has 

 succeeded in straining alcohol at the ordinary temj^erature with a pull 

 equivalent to a negative pressure of 25 atmospheres, by completely 

 tilling a bulb with alcohol and then cooling it. The alcohol in con- 

 tracting strains the bulb inwards, and finally, when the tension 

 becomes very great, parts from the glass with a sharp " click." 



To realise 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 without condensation. 

 But a glance at the figure will render the matter plain. Consider 

 the isothermal (175') 75^ for ether at the point marked B. If it 

 were possible to lower the temperature to 160^ without condensation, 

 keeping volume constant, pressure ^vould fall, and the gas would 

 then be in the state represented on the isothermal line 160^ at G ; 

 that is, it would 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 air be suddenly expanded 

 it will do work against atmospheric pressure, and will cool itself. 

 This globe contains air ; but the air has been filtered carefully through 

 cotton-wool, with the object of excluding dust-particles. It is satu- 

 rated with moisture. On taking a stroke ol the pump, so as to 

 exhaust the air in the globe, no change is evident ; no 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 the 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 

 comparatively few dust-particles, because only a little dusty air has 

 been admitted. On again repeating, the fog is denser; there are more 

 particles on which moisture 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 one foot ; that ol the metric system is one gramme 



