182 HEAT. 



how it is so without any appeal to that equation. If we take a quantity 

 of steam at 100 and 760 mm., and gradually alter the pressure to 787 "63, 

 not allowing any heat to escape, the work done in the compression goes 

 to heat the gas, and its specific heat is such that the temperature rises 

 above 101. To bring it back to 101, heat must be abstracted. Or, 

 putting it in another way, when in the compression the temperature 

 is 101, the pressure is less than 787-63, the condensing pressure. 

 During the rest of the compression, to keep the temperature at 101, we 

 must let heat escape. The specific heat, therefore, of saturated steam is 

 negative. The total energy supplied is not negative, for the positive 

 work done on each gramme is in excess of the mechanical equivalent of 

 the negative specific heat. 



If, therefore, we have a quantity of steam near its condensing point, 

 and we compress it without allowing heat to escape, its temperature rises, 

 and it gets further and further away from the condensing point. On the 

 contrary, if we reduce the pressure it comes nearer to its condensing point, 

 and, with sufficient reduction, we have a cloud formed. This cooling ac- 

 companying diminution of pressure explains a common formation of clouds 

 in the atmosphere. This mode of cloud-forming is especially noticeable 

 near the summits of mountains, when a current of air blows up the moun- 

 tain side into regions of diminished pressure. It there expands, parting 

 with energy to push out the surrounding air, and so cools. The water- 

 vapour in the air at the same time expands, cools, and, at last, condenses. 

 The same effect is also seen during the formation of thunder clouds. Large 

 masses of damp and, therefore, light air rise, and cool through expansion 

 against diminished pressure. The temperature falls below that for which 

 the new pressure of the vapour is the maximum, and condensation occurs, 

 forming the large cumulus masses high up in the air so characteristic of 

 thunder clouds. 



Other saturated vapours, among them alcohol, agree with water in 

 having this so-called negative specific heat, so that condensation will 

 occur with rarefaction. Some saturated vapours, such as that of ether, 

 possess a positive specific heat, and these on sudden compression tend to 

 condense. For, taking saturated ether-vapour at t, and raising the 

 pressure to that corresponding to saturation at t+ 1, the temperature is 

 not so high as t+ 1, but say t + 6, and heat is necessary to raise the 

 temperature from t + 6tot+I. But if this heat is not supplied, the 

 pressure is above the maximum at t + 6, and condensation occurs and 

 continues until the latent heat yielded raises the whole to t + 1. 



Spheroidal State. 



When a metal plate is heated nearly to rednesss, and a few drops of 

 water are thrown upon it, the drops rolls about on the plate without 

 spreading out, and without boiling. They are then said to be in the 

 spheroidal state. If the plate is a flat one, by looking at it edgeways it 

 is possible to see that the drops are not in contact with the plate. 



It is easy to show this also by bringing a wire nearly in contact with 

 the plate, and connecting the wire with the plate through a battery and 

 galvanometer as in Fig. 106, where the plate is the top of an inverted 

 platinum crucible. Allowing a little liquid to trickle down the wire 

 it assumes the spheroidal condition when it approaches the plate, 



