CHANGE OF STATE LIQUID VAPOUR. 



179 



boiled by a ring-burner, or otherwise. The steam issues from the lower 

 end of the tube. A calorimeter, containing a known weight of water 

 at a known temperature, is then brought under the flask, so that the 

 steam passes into the water and is condensed there. Allowing the con- 

 densation to proceed for a short time, the calorimeter is then withdrawn 

 and the temperature noted. Its increase in weight gives the quantity 

 of steam condensed. Then, on the one hand, we have the gain of 

 heat by the calorimeter and its original contents, and, on the other 

 hand, the latent heat given up by each gramme of steam in condens- 

 ing, together with the heat given up in falling from the temperature of 

 boiling to the final temperature of the calorimeter. 



Equating these, we can determine the latent heat. 



For example, a calorimeter whose equivalent was 10 gms. contained 

 140 gms. of water at 15 tempera- 

 ture. Steam at 100 was passed 

 into it until the temperature rose 

 to 75, that is, through 60. The 

 contents of the calorimeter weighed 

 now 157 gms., so that- the total 

 quantity of steam condensed was 17 

 gms. If the latent heat given up 

 by each gramme is L, it may be 

 regarded as giving up L and then 

 25 more calories in falling to 75. 

 We therefore have the equation 



150 x 60 = (L + 25)17, 

 whence L = 504. 



Boiler 



Burner 



Calorimeter 



FIG. 104. Rough Determination 

 of Latent Heat. 



This experiment, of course, is affected 

 by very serious errors. Heat is in- 

 troduced not only by the steam but 

 also by the conducting pipe, and the 

 amount so introduced must be deter- 

 mined and allowed for. Heat is, on 



the other hand, lost by conduction or radiation from the calorimeter, and 

 its amount must also be determined and allowed for. In the case of 

 water, we may allow the steam to mix with the water, though unless we 

 measure the temperature of the steam directly, a small error comes in 

 through the additional pressure of the steam and the consequent rise of 

 the boiling-point, owing to the mouth of the steam-pipe being below the 

 level of the water in the calorimeter. But in the case of other liquids, 

 we cannot have mixture with the water in the calorimeter. It is better, 

 therefore, to have a spiral condensing worm in the calorimeter, ending in 

 an enlargement to collect the condensed liquid. The arrangement 

 adopted by Berthelot (Fig 105) will illustrate this. 



F is the vessel containing the boiling liquid, T the vapour-pipe, OSR 

 the condensing worm, I a ring-burner under a gauze, the calorimeter 

 being protected against radiation from the flame by a non-conducting 

 cover faced with metal nn. With this apparatus Berthelot found the 

 latent heat of steam at 100 0. to be 536*2 (the mean of three determina- 

 tions, 535-2, 537*2, 536 - 2), agreeing nearly with Regnault's determination, 



