136 Mr, Rainy on tite [Aug. 



One of the most important of these experiments is related in 

 vol. i. p. 67 to 76 ; and is intended to prove that the specific 

 spravily o{ dry hydrogen is exactly ^'.j. of that o^dry oxygen ; but 

 Dr. Thonuson appears to me in this case to have been led into a 

 very cousiderable error, by urder-rating the quantity of vapour 

 in the hydrogen. The hydrogen was disengaged at temperature 

 49°, at which, according to Dalton's table, the tension of vapour 

 = 0*363 inch. Dr. Thomson supposes the specific gravity of 

 vapour at 49° to be '00533 compared with dry air at 60°, and 

 mider a pressure of 30 inches. It is easy to show, however, 

 that '00633 is nearly the specific gravity of vapour at temperas- 

 tare 212°, and pressure 0*363 inch ; and that the specific gravity 

 of vapour at temperature 49® and pressure 0*363, is considerably 

 greater. 



If the specific gravity of dry air at temperature 60° and baro- 

 meter 30 = 1 ; the specific gravity of vapour at temperature 

 212° and barometer 30 will be 0*481, and the specific gravity 



of vapour at 212° and barometer 0-363 will be 0*481 x ^^ = 



0*00582. To find from this the specific gravity of vapour at 

 tension 0*363, and temperature 49", we must consider that the 

 vapour, if reduced in temperature from 2 12° to 49°, will, without 

 condensing into a liquid, be reduced in bulk, like any of the 

 gases, from 660 parts to 497 ;* and consequently its specific 



gravity will increase from 0*00582 to 0*00582 x ^ = 0*00772, 



which is the true specific gravity of vapour at temperature 49°, and 

 at the corresponding tension0'36d. 



Both Dr. Apjohn and Dr. Thomson have given erroneous 

 formula; for calculating the specific gravity of aqueous vapour, 

 founded on the supposition, that if we take vapour at 212 and 

 barometer 30 as the standard, the density of vapour fl^ any other 

 temperature is exactly proportional to the pressure which it 

 supports, without any reference to the temperature.f This 

 opmion is quite inconsistent with the properties of vapour, as is 

 evident from the illustration which I have just given; it is also 

 inconsistent with Gay Lussac's theory of volumes. Several 

 years have now elapsed since that gentleman has shown, that a 

 volume of aqueous vapour (of any tension and temperature) con- 

 sists of one volume of hydrogen and half a volume of oxygen at 

 the same tension and temperature. This is true at 212°, but it 

 is equally true at 49° or any other temperature. The specific 

 gravity of aqueous vapour is to the specific gravity of atmo- 

 gpheric air always as 0*625 to 1, the temperature and pressure 



• Dalton and Gay liUisac have shown that 480 volumes of a gas at temperature 32<>, 

 txpand to 497 at 49", and to 66() at "Zl'i^. The same h true of vapours if not in con- 

 tact with their liquids. 



f Arinaix qf VhUofophy, N. 6. vol, iii. p. S05 and 366. 



