﻿168 . Mr. C. Tomlinson's Historical Notes on some 



13. Attention was also called to a conclusion arrived at by 

 (Ersted*, that gas resulting from chemical decomposition is never 

 disengaged from a liquid except in contact with some solid body ; 

 and he proposes to apply this principle to the disengagement of 

 vapours. He says: — "If a metallic wire be suspended in a 

 boiling fluid, it instantly becomes covered with bubbles of va- 

 pour." To show the application of this fact, ten pounds of brass 

 wire, one-fifth of a line in diameter, were put into a still contain- 

 ing ten pints of brandy ; seven pints came over at a heat which 

 without the wire would have sent over only four. 



14. In 1835 Le Grandf remarked that " when water is boiled 

 in a glass vessel the boiling goes on at first regularly, with the 

 production of numerous bubbles, and without noise ; but as soon 

 as the water has lost the greater part of the air that it held in 

 solution matters become changed; the ebullition takes place in 

 intermittent bursts, accompanied with noise, and the thermometer 

 experiences considerable oscillations. This is known as bump- 

 ing. Many salts in small quantity prevent it in a remarkable 

 manner. Others favour it highly, especially the neutral tartrate 

 of potash. Platinum is generally used to prevent it, on account 

 of its unalterable character; but this is a mistake; for the mo- 

 ment a small quantity of platinum-filings is added, ebullition is 

 facilitated, on account of the air introduced; but as soon as this 

 air is disengaged the bumpings are resumed." 



15. In 1837 some curious experiments were recorded by 

 Schonbein J, in which the effect of gas generated in the midst of 

 water near the boiling-point was shown. For this purpose 1 per 

 cent, of sulphuric acid was added to the water, and when it was 

 boiling the lamp was removed and some bits of iron or zinc wire 

 were thrown into the flask. Ebullition set in again immediately, 

 and continued during some seconds. If the wire was first rolled 

 up into a coil before being thrown in, the effect was so consider- 

 able that a portion of the contents of the flask was thrown out. 

 Metals that do not disengage hydrogen, and from whose surface 

 the adhering film of air has been removed by dipping them into 

 boiling water (such as platinum, gold, silver, &c), will not dis- 

 engage any vapour from water near the boiling-point. 



16. Schonbein's explanation of the effect of zinc in the acid 

 water did not depend^ as we should have supposed, on the effect 

 of air or gas in diminishing the cohesive force of the liquid par- 

 ticles, but his theory rested entirely on the principle of heat 

 overcoming pressure. He reasons in this way : — Hydrogen on 

 being liberated has the same pressure as the atmosphere ; the 



* Gehlen's Journal fur Chemie, vol. i. pp. 277-289. 

 f Ann. de Chim. et de Phys. vol. lix. p. 426. 

 % Pogg. Ann. vol. xl. p. 391. 



