54 THE NATURE AND [SECT. 11. 



On determining, by calculation, the quantity of water which may be heated 

 one degree, by the heat developed in the condensation of the vapour, he took care 

 to keep an account of the difference of the capacity of water for heat from that of 

 alcohol. l 



The result of Count Rumford's calculation is nearly the same as by the formula, 

 (art. 75, note,) when we assume the specific heat of the alcohol vapour and liquid 

 to be the same, and equal to 58. Thus from the second experiment 



42909_x 10 + (>5g x Q.^ = ^.^ 



from whence, deducting 173 x -58 for the heat due to the temperature of the 

 vapour, we have 506 nearly, for the heat of conversion from liquid to vapour. 

 The Count's number is 500-03. 



Count Rumford also ascertained that the vapour of sulphuric ether afforded 

 only about half the heat in condensation that alcohol afforded, or one-fourth of 

 the heat furnished by condensing the steam of water. 



81. Important as a knowledge of the heat of conversion into vapour is, it was 

 not further investigated till 1817, when Dr. Ure made a few experiments on diffe- 

 rent bodies. 2 His mode of experiment was exceedingly simple. The apparatus 

 consisted of a glass retort of very small dimensions, with a short neck, inserted 

 into a globular receiver, of very thin glass, about three inches in diameter. The 

 glass was fixed steadily in the centre of 32340 grains of water, at a known tempe- 

 rature, contained in a glass basin. Of the liquid, whose vapour was to be 

 examined, 200 grains were introduced into the retort, and rapidly distilled into the 

 globe by the heat of an Argand lamp. The temperature of the air was 45, that 

 of the water in the basin from 42 to 43 ; and the rise of temperature occasioned 

 by the condensation of the vapour never exceeded that of the air by four degrees. 

 As the communication of heat is very slow between bodies which differ little in 

 temperature, the air could exercise no perceptible influence on the water in the 

 basin during the experiment, which was always completed in five or six minutes. 

 A thermometer of great delicacy was continually moved through the water, and its 

 indications were read off, by the aid of a lens, to small fractions of a degree. 



The distillation was rapidly performed ; and we are assured by Dr. Ure, that in 

 the numerous repetitions of the same experiment, the accordances were excellent. 

 The following table gives the mean result, the last column being calculated by the 

 formula, (of note to art. 75.) 



i Philosophical Mag. vol. xliii. p. 67. Philosophical Transactions for 1818. 



