RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 



523 



of four or five centimeters, the sil- 

 vering being on both walls, so that 

 any heat passing from the boiling 

 liquid to the calorimeter water by 

 radiation had to pass through two 

 brightly silvered surfaces and a 

 vacuum space. The conduction of 

 heat to the calorimeter through the 

 glass itself cannot be prevented, but 

 was made small by having the glass 

 as light as was consistent with the 

 strength demanded. The proper 

 correction was always applied for 

 heat gained by the calorimeter in 

 these two wa3''s, the necessary ob- 

 servations being always determined 

 before each measurement of latent 

 heat. For several minutes previous 

 to admitting the vapor into the 

 condenser, readings of the temper- 

 ature were made at intervals of 

 one minute, until the increase be- 

 came constant and of certain value. 

 The nature of the problem is such 

 as to make this correction abso- 

 lutely necessary, for the vaporizer 

 must be brought close to the calo- 

 rimeter water, and radiation and 

 conduction across this small dis- 

 tance cannot be prevented. 



After many experiments had 

 been made, not only with water 

 but also with higher boiling sub- 

 stances, the accidental cracking of 

 the vaporizer jacket near the top 

 destroyed its high vacuum ; nev- 

 ertheless the correction for the 

 combined radiation and conduc- 

 tion was then found to be but little 

 greater than before. Apparently 

 the two brightly silvered surfaces 

 effectually prevented radiation. 



Figure 4. Final Form of Vaporizer 

 (3^ actual size). 



A vacuum jacket (AB), silvered in- 

 side, surrounds the hot vessel, which is 

 provided with a stopcock above. The 

 delivery tube has two traps, one over 

 its top (C), and another (E) as near as 

 possible to the water of the calorimeter 

 below. The condenser is attached at the 

 very bottom at G, and F is the water- 

 line, as in Figure 3. 



