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



sulphuric acid the boiling-point of water rose to 106° C; so 

 much had the internal surface of the glass become modified by 

 the acid; but by simply heating the flask to 300° or 400° C. a 

 similar result was obtained. The conclusion arrived at was that 

 " the effect was due to a species of molecular modification of the 

 surface of the glass, brought about by the acid, of such a nature 

 as to increase the adhesion of the water and so delay its boiling." 

 In other words, the glass was made chemically clean. Still, 

 however, the author does not catch this idea ; he goes on con- 

 triving new experiments which again bring him u to some slight 

 modification in the physical texture of the glass by the action of 

 the sulphuric acid, or by destroying the impalpable dust retained 

 between the molecules of the glass. Anew flask, however clean 

 in appearance, always has more or less of such dust on its sur- 

 face. This kind of dust or varnish (vernis) not only adheres 

 to the surface of glass, but forms part of its very substance, 

 imprisoned between its molecules, and the adhesion of glass for 

 water is diminished thereby/-' Heating the glass to redness 

 " may destroy foreign matter on the glass, or up to a certain 

 point modify the molecular condition." In the last page of his 

 memoir Marcet is still referring to these " molecular changes 

 (so powerful was the influence of Gay-Lussac on scientific opi- 

 nion) (10); and it is only in summing up the conclusions " drawn 

 from his inquiry, still very incomplete (encore fort incomplet), that 

 he makes use of the expression " perfectly clean glass vessels.''* 

 In them, he says, '/water and alcohol may be raised to a higher 

 temperature than has hitherto been known." 



20. In 1844 Magnus*, in the course of some observations on 

 the elasticity of steam, has some remarks on the cohesion and ad- 

 hesion of water during the boiling. If the walls of the vessel had 

 an infinitely stronger adhesion for the water than the liquid par- 

 ticles had of cohesion among themselves, or if the walls of the 

 vessel were of water, we might obtain a measure of the cohesive 

 force, because in such case the water would only attain a boiling- 

 point at which the elasticity of the steam was sufficient to overcome 

 the pressure of the air and thecohesion of the water. But in a vessel 

 whose sides have less adhesion for the water than the liquid mole- 

 cules have of cohesion among themselves, a less force is required 

 to overcome the adhesion than the cohesion. Hence the boiling- 

 point is lower in proportion as the adhesion of the sides or of 

 any foreign insoluble substance in the water is small. It is sup- 

 posed that a smooth metal surface to which the water-particles 

 adhere more strongly than they cohere, must raise the boiling- 

 point — although experiment shows the contrary. If a perfectly 

 clean metal surface be dipped into water so as to wet it com- 

 * PoggendorfFs Annalen, vol. lxi. p. 248. 



