98 Dr. A. Waller's Observations on certain 



capable of forming traces has the property of disengaging its 

 gas in bubbles, whenever it is brought in contact with any dry 

 surface ; as for instance, when a mixture of this sort formed 

 on a slip of glass is caused to spread over a part of the surface 

 which has not previously been wetted, bubbles of gas are im- 

 mediately evolved on that spot, although none are perceived 

 elsewhere. This effect is also produced with champagne, 

 seltzer and other effervescing waters, which however have not 

 the property of forming gaseous traces. Any surface, whether 

 metallic or non-metallic, will be found to effect the separation 

 of the gas from the liquid; and I have not perceived that 

 there- was any difference from the surface being perfectly po- 

 lished or rough. 



The immersion of a piece of bread in champagne to renew 

 the effervescence, is merely an example of the contact of a 

 fresh surface with the gas; in a short time it ceases to have 

 this effect, but if a fresh piece is used, the effervescence is re- 

 newed as before. The difference of effect between this and a 

 piece of metal arises solely from the superior extent of surface 

 presented by the cavities of the bread. The disengagement 

 of steam from boiling water by platinum foil or any other solid 

 substance, is likewise of the same nature. After a very short 

 time this effect ceases, unless renewed by a fresh surface. 

 The most natural explanation of these phaenomena, is to refer 

 them to some molecular action of the solid on the gas, proba- 

 bly of a mechanical nature, which lasts a very short time, when 

 the solid acquires a "droit de domicile" in the liquid, and be- 

 comes perfectly inert. M. Legrand, who has made most cor- 

 rect experiments on the point of ebullition of saline solutions, 

 remarks, that platinum possesses no power in equalizing ebul- 

 lition after a few moments, when, according to him, all the air 

 has been expelled from its surface ; but on the contrary, zinc 

 and iron will act as long as they are present in the liquid, 

 which he attributes to their power of decomposing water. 



Previously to showing the existence of the same action in 

 bodies in a state of vapour or of fume, I will make a short di- 

 gression with respect to the constitution of vapours in general. 



The term vapour is commonly applied to bodies in three 

 different conditions, — 1st, that of temporary gas diffused in 

 the atmosphere; 2nd, that of liquid particles mechanically 

 suspended there ; 3rd, that of solid particles suspended in like 

 manner. To the two latter, to speak more correctly, may be 

 applied the term of fumes. The first correspond to solution 

 in a liquid, and the other two to that of suspension in the 

 same. As examples of the first, we have the vapours of water 

 while in an invisible state, and those of bromine, &c. Of the 



