188 Mr. Rainy's Reply to Dr. Thomson. [March, 



thesis. I endeavoured to show that when a proper correction 

 was made for the influence of vapour, the experiments indicated 

 that the specific gravity of oxygen is to that of hydrogen as 

 16'54 to 1, and therefore are opposed to Dr. Front's views. In 

 an answer to my paper (New Series, vol. x. p. 352), Dr. Thom- 

 son states that the dilute acid which he employed for disen- 

 gaging his hydrogen consisted of 1700 grains of water to 

 400 grains of sulphuric acid, that its boilmg point was 224°, 

 and, therefore, that hydrogen evolved from it at 49° would con- 

 tain vapour of a tension not exceeding that from pure water at 

 37**, which by Dalton's table is 0*237. If the correction for 

 vapour is made on these data, Dr. Thomson shows that it will 

 follow from his experiments that the specific gi-avity of oxygen 

 is to that of hydrogen as 16 to 1-0077. In the detail of the 

 experiment given in the Principles (vol. i. p. 70), after mention- 

 ing that the experiment was performed at temperature 49°, Dr. 

 Thomson states expressly that " the specific gravity of vapour 

 at 49° is 0"00533." I certainly did not suppose that by vapour 

 of 49°, Dr. Thomson could mean vapour of temperature 49°, but 

 of a tension corresponding to 37°, especially as 0*00533 is pretty 

 nearly the specific gravity of vapour at 49°, if calculated by a 

 rule which Dr. T. himself has given in the Annals (New Series, 

 vol. iii. p. 305). I did think it likely that the tension of vapour 

 from the dilute acid would be somewhat less than from water ; 

 but as Dr. Thomson had not stated the strength of his acid, I 

 had no means of estimating the difference, and from his own 

 words above quoted, I supposed that he considered the difference 

 as too minute to require particular notice. 



I believe considerable doubts are now entertained respecting 

 the correctness of the method of estimating the tension of vapour 

 from a liquid by comparing its boiling point with that of another 

 liquid, of which the tension at the given temperature is previously 

 known. This is the method adopted by Dr. Thomson. I 

 thought it would be satisfactory to take the tension of the dilute 

 acid directly. With this view I prepared a mixture, containing 

 by weight 1700 parts water to 400 of the strongest liquid sul- 

 phuric acid. This mixture had the specific gravity 1*157, and in 

 various careful trials, I found its tension at 4y° tobeO-32, instead 

 o/ 0-237 as calculated by Dr. Thomson. In order to find its 

 boiling point, I heated it in a platina crucible over a lamp. It 

 boiled briskly at 217°. I kept it boiling till its temperature rose 

 to 224°. Being covered up and cooled, 185*24 grains of it 

 dissolved 39 grains of zinc ; whence if we admit with Dr. Thom- 

 son that the atoms of zinc, sulphuric acid, and water, are 

 respectively ''s the numbers 4*25, 5 and 1*125, it will follow that 

 the acid which boils at 224° consists of 400 anhydrous sulphuric 

 acid to 1210 water, or of 400 strongest hquid acid to 918 water. 

 Dr. ThomsoQ has not stated whether his acid consisted of 1700 



