"Inactive" Condition of Solids. 141 



will have its surface deprived of air, and so will not be in a con- 

 dition to liberate gas. Let us see. 



Exp. 9. A glass rod that had long been exposed to the air 

 was put into olive oil, and this was raised to the temperature 

 of 310° F. It was left in the oil about twenty minutes, then 

 taken out and passed hastily through a duster to get rid of the 

 excess of oil, and so plunged into soda-water. It was instantly 

 and completely covered with bubbles, — so much so, that no part 

 of the immersed portion was free from them. 



Here the rod had not only lost its supposed film of air, but 

 was covered with a thin layer of oil ; it was, in fact, about as 

 chemically unclean as it well could be, and in this state it exhi- 

 bited the strongest adhesion for air, and little or none for water. 



I cannot help thinking that in examining the phenomena in 

 question sufficient attention has not been bestowed on the re- 

 markable difference between a chemically clean and an ordina- 

 rily clean surface. A body that is capable of performing cer- 

 tain offices at one time becomes incapable of doing so at another, 

 and the circumstances which lead to the altered condition are 

 apparently insufficient to produce such marked differences. This 

 has thrown an air of mystery over certain phenomena which seem 

 to me to admit of very simple explanation. I believe that most, 

 if not all, of the phenomena connected with the crystallization 

 of supersaturated solutions depend on this principle of adhesion ; 

 but I propose to refer to this subject on another occasion. I must, 

 however, insist on the necessity of distinguishing between the 

 presence or absence of the condition of chemical purity of a sur- 

 face as the key to the explanation of a large class of phenomena. 

 I must further remark that it is difficult to maintain the condition 

 of chemical purity for any length of time in apparatus with which 

 one is working. A slight touch, an open door, a puff of smoke, 

 a draught bringing dust is often sufficient to disturb it, as, for 

 example, — 



Exp. 10. A thick iron wire put into soda-water caused a co- 

 pious disengagement of gas. The wire was wiped with a clean 

 duster, it still produced the same effect ; it was carefully po- 

 lished with sand-paper, and still the same effect ; it was dipped 

 into spirit and washed in water, and when placed in soda-water 

 not a bubble appeared on its surface; it was then wiped with 

 what is usually called a clean duster, and on putting it into 

 soda-water it immediately disengaged bubbles. 



Exp. 11. A large fragment of flint disengaged gas from every 

 part of its surface. It was broken into two pieces, and again 

 put into the soda-water. Not a bubble of gas was liberated 

 from the two fresh surfaces. 



It may be objected to this experiment that the new surfaces 



