130 M. Gay^Lussac on the Boiling Point of Fluids. [AuqusT, 



facts. When a mattrass half filled with water is made to boil, 

 a considerable noise is produced, which indicates that the ebulli- 

 tion takes place with some difficulty ; large bubbles of vapour 

 are formed, which rise up from certain points only, and a ther- 

 mometer plunged into the fluid experiences frequent variations. 

 If we employ a vessel of tinned iron, the noise is less consider- 

 able, the bubbles are smaller, but more numerous, the variations 

 of the thermometer are less considerable, and the boiling point 

 is lower* We may confirm this observation by boiling water in 

 a glass vessel, and throwing into it a few filings of iron, when 

 the ebullition will immediately proceed in the same manner as 

 in a metallic vessel. If we employ sulphuric acid instead of 

 water, the difference is more considerable, amounting often to 

 many degrees. 



When a fluid is boiled in glass, not only is the ebullition more 

 slow, but a thermometer plunged into the fluid experiences con- 

 siderable variations, and rises above the real boiling point. It 

 is supposed that the cohesion or viscidity of a fluid must have 

 a considerable effect upon its boiling point; for the vapour 

 which is formed in the interior of a fluid has two forces to over- 

 come ; the pressure upon its surface, and the cohesion of the 

 particles. It may be supposed that a solid or a fluid body, the 

 vapour of which is formed at its surface, may be in a state 

 of equilibrium with the pressure of the atmosphere, while the 

 interior portion may acquire a greater degree of heat than that 

 of the real boiling point, provided the fluid be confined in a 

 vessel and heated at the lower part, as generally takes place in 

 the boiling of fluids. In this case the adhesion of the fluid to 

 the vessel may be considered as analogous to its viscidity. 



The disengagement of an elastic fluid, which is dissolved in 

 water, is analogous to the ebullition of a fluid. If we take any 

 brisk fermenting liquor, and wait until the escape of the gas 

 has ceased, it may be renewed by introducing into it any solid 

 substance, as a piece of paper, a crust of bread, a powder, or 

 even by agitation. The carbonic acid is disengaged principally 

 where the fluid touches the glass, and particularly at any part 

 where there are asperities in the surface. 



On this principle we explain the sudden starts which some- 

 times take place in the boiling of fluids. When by any means 

 the temperature of a fluid is raised above the true boiling point, 

 it happens that some change occurs, by which a new state of 

 things is induced, and the superfluous heat is suddenly disen- 

 gaged in the form of a great rush of steam. This frequently 

 occurs to a great degree in distilling sulphuric acid, by which 

 the vessels are not unfrequently broken, when they are of glass ; 

 this evil may be effectually obviated by putting into the retort 

 some small pieces of platina wire, when the sudden disengage- 

 ment of gas will be prevented, and consequently the vessels will 

 not be liable to be broken. 



