on Gaseous Supersaturated Solutions. 281 



Henrici, was absolutely passive, even when set in motion so as 

 to expose fresh surfaces to the solution. 



When a surface is properly cleaned by one or other of the 

 methods pointed out, the test of the cleansing process is so deli- 

 cate, that on immersing the solid in soda-water the greater or 

 less appearance of the bubbles or their entire absence marks 

 exactly how much, or how little, or how completely the pro- 

 cess has been successful. For example, the middle portion of 

 a thick platinum wire was held in the flame of a spirit-lamp 

 until it glowed. It was then put into soda-water, when there 

 was an abundant deposit of bubbles above and below the part 

 that had been heated, but not a bubble on that part. A glass 

 tube was dipped into sulphuric acid to the depth of about 2 

 inches ; a cork was then driven into the upper end, and the tube 

 sunk to the depth of 4 inches in the acid. It was then taken 

 out and rinsed, the cork being removed, and so placed in soda- 

 water to the depth of 6 inches. 4 inches of the outside and 2 

 inches of the inside were free from bubbles ; but the remainder 

 of the tube, both inside and out, was thickly coated with them. 

 A glass cylinder 7 inches in height, that had been made clean 

 by the action of strong sulphuric acid, was wiped with what 

 would be called a clean duster to the depth of about 2 inches at 

 the open end, and then filled up with soda-water. The part that 

 had been wiped was accurately delineated by being covered with 

 bubbles, while the remaining 5 inches of surface were com- 

 pletely free from them. In a clean glass full of soda-water the 

 finger was introduced below the surface with friction against the 

 side. The finger-mark became immediately apparent in conse- 

 quence of the formation of bubbles upon it. It was formerly 

 supposed that rough surfaces were particularly favourable to the 

 liberation of gas. Such surfaces have really no action, provided 

 they are chemically clean and not porous, as was shown in my 

 former experiments in the case of a ratVtail file*. But it may 

 happen that in attempting to clean an iron or steel wire, such 

 as a knitting-needle, in strong nitric or dilute sulphuric acid, 

 the surface becomes graphitic ; and in such a case, on placing it 

 in soda-water, it becomes immediately covered with bubbles. 



Dr. Henrici found that quartz-sand gave off numerous bubbles 

 of gas — a clear proof that it was unclean. He also 'found that 

 charcoal made red-hot and plunged into soda-water was inactive, 

 doubtless from the absorption of gas. I obtained the same re- 

 sult some years ago, and also that cocoa-nut-shell charcoal and 

 boxwood-charcoal by long keeping under water that had been 

 boiled, were equally inactive in soda-water, since the pores were 

 already filled, or the gas contained in them had entered into 

 * Phil. Mag. for August 1867. Experiments 3 and 4. 



