on Gaseous Supersaturated Solutions* 277 



worked in the same field and published the results of my labours 

 in this Magazine*, it may not be thought out of place if I here 

 offer some opinions on Dr. Henri ci*s results, which stand out, as 

 he admits, in singular contrast to my own. 



In an inquiry of this kind it is, of course, necessary to define 

 accurately what is meant by (1) a supersaturated gaseous solu- 

 tion, (2) a chemically clean surface in contradistinction to (3) one 

 that is not clean. I do not object to Dr. Henrici's statement as 

 to the first ; but he seems to me to have but an imperfect idea as 

 to the second. Having succeeded in making metallic surfaces 

 sufficiently clean for voltaic contact by rubbing them with finely 

 powdered pumice-stone spread on leather, he adopts this same 

 process for making clean the glass rods, metallic wires, and some 

 of the other bodies used in his inquiry; and he objects to one 

 of the methods used by me for the purpose, namely heating 

 them in the flame of a spirit-lamp, on the ground that the heat, 

 instead of getting rid of the impure film that covers them more 

 or less, simply converts that film into a porous body which still 

 invests the surface. 



The method of cleaning the wires &c. by means of pumice- 

 powder spread on leather is absolutely of no value with reference 

 to the present inquiry. The surfaces thus treated are still un- 

 clean, and naturally become covered with bubbles on immersing 

 them in soda-water &c. So also in the application of heat, Dr. 

 Henrici made use of a small alcohol-flame, and passed the bodies 

 over and above it rather thaii through it; whereas he should 

 have made his platinum wires red-hot and even white-hot. 



In several of my published papers f I have endeavoured to 

 show that the obscure and often contradictory behaviour of 

 solids as nuclei, in separating gas, or vapour, or salt from their 

 supersaturated solutions, becomes clear by considering whether 

 the solids used as nuclei, or the walls of the flasks, test-glasses, 

 and other containing vessels were or were not chemically clean 

 as to surface at the moment of contact with the solution. 



A body was defined as chemically clean the surface of which 

 is entirely free from any substance foreign to its own composi- 

 tion. And it will be remarked that I speak of surface only. A 

 glass rod is chemically clean, although a particle of carbon, or of 

 oxide of iron, or other matter be enclosed and shut up within it ; 

 but not so if that particle reach and form a portion of the sur- 

 face itself. So also a stick of tallow, stearine, paraffin, resin, 

 sulphur, &c. is chemically clean so long as its surface falls under 

 the definition just given. 



* Phil. Mag. for August and September 1867. 



t Some of the definitions given in the text are from the Philosophical 

 Transactions for 1871, p. 51. 



